Exhaust gas purification catalyst

ABSTRACT

The present invention is directed to address the following problem: in an exhaust gas purification catalyst comprising a dual catalyst of a combination of a startup catalyst and an underfloor catalyst, reduction in the gas diffusivity of the underfloor catalyst results in reduction in the use efficiency of a catalytic active site, resulting in reduction in purification performance. The present invention relates to an exhaust gas purification catalyst comprising a dual catalyst of a combination of a startup catalyst and an underfloor catalyst having a catalyst coating where a large number of voids are included, wherein high-aspect-ratio pores having an aspect ratio of 5 or more account for a certain rate or more of the whole volume of the voids, to thereby enhance the purification performance of the catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2016/059358 filed Mar. 24, 2016, claiming priority based onJapanese Patent Application No. 2015-065688 filed Mar. 27, 2015, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification catalyst.More specifically, it relates to an exhaust gas purification catalystcomprising a dual catalyst of a combination of a startup catalyst and anunderfloor catalyst, wherein the underfloor catalyst comprises acatalyst coating layer having high-aspect-ratio pores at a certain rate.

BACKGROUND ART

Exhaust gas discharged from an internal combustion engine of anautomotive or the like includes harmful gases such as carbon monoxide(CO), nitrogen oxides (NOx), and unburned hydrocarbon (HC). An exhaustgas purification catalyst for decomposition of such harmful gases isalso referred to as a “three-way catalyst”, and commonly has ahoneycomb-shaped monolith substrate made of cordierite or the like and acatalyst coating layer formed thereon by wash coating of a slurryincluding a noble metal particle having catalyst activity and anauxiliary catalyst having oxygen storage capacity (OSC).

Various approaches have been made in order to enhance purificationefficiency of the exhaust gas purification catalyst. There is known, forexample, a procedure where a void is formed in a catalyst coating layerin order to enhance diffusivity of exhaust gas in the catalyst coatinglayer. For example, known methods for forming a void in a catalystcoating layer involve increasing the particle size of a catalystparticle, or use of a pore-forming material which disappears in firingof a catalyst at the final stage of production to provide a void. Forexample, Patent Literature 1 describes a method where a void is providedby adding magnesia having a particle size of 0.1 to 3.0 μm to form acatalyst layer.

If a void is provided in a catalyst layer, however, the thickness of thecatalyst layer is increased due to the void, and therefore the pressureloss of the catalyst may be increased to cause engine output powerand/or fuel efficiency to be lowered. In addition, the void provided byany of the above methods has the following problem, for example: thestrength of the catalyst layer is decreased, or a sufficient effect isnot obtained because of poor void linkage. In view of such a problem,for example, Patent Literature 2 describes a method where a carboncompound material having a predetermined shape is mixed and is allowedto disappear in catalyst firing to thereby provide a void in a catalystlayer, the void having a mode in the frequency distribution with respectto the depth to length ratio (D/L) in the cross section of 2 or more.

CITATION LIST Patent Literature

-   Patent Literature 1: JP Patent Publication (Kokai) No. 2010-104897 A-   Patent Literature 2: JP Patent Publication (Kokai) No. 2012-240027 A

SUMMARY OF INVENTION Technical Problem

In recent years, an exhaust gas purification catalyst which comprises adual catalyst of a combination of a startup catalyst (also referred toas S/C, startup converter, or the like) and an underfloor catalyst (alsoreferred to as UF/C, underfloor converter, or the like) has beenfrequently adopted. In such a dual catalyst system, UF/C is provided atthe downstream of S/C, and serves to purify low-concentration exhaustgas that cannot be purified in S/C. The catalyst performance of UF/C,which purifies low-concentration exhaust gas, is controlled by gasdiffusion rate. Therefore, when diffusivity of exhaust gas in UF/C islow, the following problem is caused: the catalyst performance of UF/Cis reduced according to reduction in the use efficiency of an activesite of UF/C. In particular, such a problem is caused under a conditionof a high intake air mass in acceleration or the like (under a conditionof a high intake air mass or a high Ga condition: being the same asunder a high space velocity or high SV condition) during the transientperiod of an air-fuel ratio (A/F).

FIG. 17 shows a relationship of the NOx conversion efficiency (%) to theUF/C volume (L) under a high Ga condition during the transient period ofA/F.

{Specification of Catalyst Used in Measurement of FIG. 17:

<Substrate>

600H/3-9R-08

<S/C>

Lower layer: Pd (1.0 g/L)/CZ solid solution (120 g/L)+Al₂O₃(30 g/L)+Basulfate (10 g/L)

Upper layer: Rh (0.1 g/L)/ZrO₂ (60 g/L)+Al₂O₃ (40 g/L)

<UF/C>

Lower layer: Pt (1.0 g/L)/CZ solid solution (60 g/L)+Al₂O₃ (20 g/L)+Basulfate (10 g/L)

Upper layer: Rh (0.1 g/L)/ZrO₂ (30 g/L)+Al₂O₃ (20 g/L)

(where CZ corresponds to a ZrO₂—CeO₂ solid solution, and 1% of La isadded to Al₂O₃)}

As can be seen from FIG. 17, it is necessary to increase the volume ofUF/C in order to satisfy catalyst performance required for UF/C.

In order to attain an increase in volume of UF/C, a catalyst coating ofUF/C in which sufficient gas diffusivity is achieved even under a highGa condition during the transient period of A/F is demanded, but nosolution has been heretofore found.

Solution to Problem

The present inventors have made studies to solve the above problems, andas a result, have found that when an organic fiber having apredetermined shape is used as a pore-forming material, a catalystcoating which has a high-aspect-ratio pore excellent in gascommunicability and is excellent in gas diffusivity can be formed.Furthermore, the present inventors have found that when such a catalystcoating is used in UF/C of an exhaust gas purification catalystcomprising a dual catalyst of a combination of S/C and UF/C, there canbe provided an increase in purification performance under a high Gacondition, in particular, during the transient period of A/F. The gistof the present invention is as follows.

(1) An exhaust gas purification catalyst comprising a dual catalyst of acombination of a startup catalyst (S/C) and an underfloor catalyst(UF/C) that is disposed posterior to the S/C in an exhaust gas flowdirection, wherein:

a catalyst coating of the UF/C is configured from one or two or morelayers;

in at least one layer of the catalyst coating of the UF/C,

-   -   an average thickness of the coating layer is in a range from 25        μm to 160 μm,    -   a porosity measured by a weight-in-water method is in a range        from 50 to 80% by volume, and    -   high-aspect-ratio pores having an aspect ratio of 5 or more        account for 0.5 to 50% by volume of the whole volume of voids;        and

the high-aspect-ratio pore has an equivalent circle diameter of from 2μm to 50 μm in a cross-sectional image of a catalyst coating layer crosssection perpendicular to an exhaust gas flow direction and has anaverage aspect ratio of from 10 to 50.

(2) The exhaust gas purification catalyst according to (1), wherein inthe catalyst coating of the UF/C, the high-aspect-ratio pore is orientedsuch that an 80% cumulative angle, in a cumulative angle distribution onan angle basis, of an angle (cone angle) between a vector in alongitudinal direction of the high-aspect-ratio pore and a vector in anexhaust gas flow direction of a substrate is in a range from 0 to 45degrees.

(3) The exhaust gas purification catalyst according to (1) or (2),wherein a 15% cumulative size, in a cumulative particle sizedistribution on a cross-sectional area basis, of a catalyst particlecontained in the catalyst coating of the UF/C is in a range from 3 μm to10 μm.

(4) The exhaust gas purification catalyst according to any one of (1) to(3), wherein in the catalyst coating of the UF/C, an amount of coatingis in a range from 50 to 300 g per liter of the volume of a substrate.

Advantageous Effects of Invention

The exhaust gas purification catalyst comprising a dual catalyst of acombination of S/C and UF/C of the present invention comprises acatalyst coating of UF/C having a high-aspect-ratio pore satisfyingpredetermined conditions to thereby have significantly enhanced gasdiffusivity of UF/C, and therefore can exhibit sufficient purificationperformance under a high Ga condition, in particular, during thetransient period of A/F.

The present application claims a priority to Japanese Patent ApplicationNo. 2015-065688, the contents described in the description, claims anddrawings of which are incorporated herein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 includes schematic diagrams illustrating one example of a FIB-SEMmeasurement method. FIG. 1(A) is a schematic diagram illustrating a partof a catalyst coating layer cross section perpendicular to an exhaustgas flow direction on the substrate of the exhaust gas purificationcatalyst of the present invention, FIG. 1(B) is a schematic diagramillustrating a test piece obtained by cutting the exhaust gaspurification catalyst in an axial direction at the position of a dottedline illustrated in FIG. 1(A), and FIG. 1(C) schematically represents anSEM image obtained by a FIB-SEM measurement method.

FIG. 2 is a scanning electron micrograph (SEM photograph) of a catalystcoating layer cross section perpendicular to an exhaust gas flowdirection on the substrate of an exhaust gas purification catalystobtained in Example 5 of Test 1.

FIG. 3 is a diagram obtained by binarization processing of the SEMphotograph in FIG. 2.

FIG. 4 is a two-dimensional projection diagram exemplifyingthree-dimensional information on a pore, obtained by analyzing acontinuous cross-sectional image of a catalyst coating layer crosssection perpendicular to an exhaust gas flow direction of the substrateof the exhaust gas purification catalyst of the present invention.

FIG. 5 is a schematic diagram illustrating a pore in the catalystcoating layer cross section at each of A to E in FIG. 4.

FIG. 6 is a schematic diagram illustrating a cone angle of ahigh-aspect-ratio pore in the two-dimensional projection diagram of FIG.4.

FIG. 7 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between theamount of coating of the catalyst coating layer and the NOx conversionefficiency.

FIG. 8 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between theaverage thickness of the catalyst coating layer and the NOx conversionefficiency.

FIG. 9 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between theparticle size of the catalyst particle and the NOx conversionefficiency.

FIG. 10 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between theporosity of the catalyst coating layer and the NOx conversionefficiency.

FIG. 11 is a graph representing a relationship between the aspect ratioand the frequency of the high-aspect-ratio pore of the catalyst obtainedin Example 5 of Test 1, and a relationship between the aspect ratio andthe frequency of the pore of the catalyst obtained in ComparativeExample 4.

FIG. 12 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between theaverage aspect ratio of the high-aspect-ratio pore and the NOxconversion efficiency.

FIG. 13 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between therate of a high-aspect-ratio pore relative to the whole of voids and theNOx conversion efficiency.

FIG. 14 is a graph representing a relationship between the cone angleand the cumulative rate of the high-aspect-ratio pore of the catalystobtained in Example 16 of Test 1.

FIG. 15 is a graph representing catalyst performance evaluation testresults of catalysts obtained in Examples 1 to 42 and ComparativeExamples 1 to 133 of Test 1, and representing a relationship between the80% cumulative angle of the high-aspect-ratio pore and the NOxconversion efficiency.

FIG. 16 represents the NOx conversion efficiency (%) of an exhaust gaspurification catalyst comprising a dual catalyst of a combination of S/Cand UF/C, prepared in Test 2, under a high Ga condition during thetransient period of A/F.

FIG. 17 represents a relationship of the NOx conversion efficiency (%)to the UF/C volume (L) of an exhaust gas purification catalystcomprising a dual catalyst of a combination of S/C and UF/C, under ahigh Ga condition during the transient period of A/F.

DESCRIPTION OF EMBODIMENTS

[Exhaust Gas Purification Catalyst]

The exhaust gas purification catalyst of the present invention is anexhaust gas purification catalyst comprising a dual catalyst of acombination of a startup catalyst (S/C) and an underfloor catalyst(UF/C) that is disposed posterior to the S/C in an exhaust gas flowdirection, and is preferably used under a high intake air mass conditionduring the transient period of an air-fuel ratio. The catalyst coatingof the UF/C has the following characteristics.

(Configuration of UF/C)

The catalyst coating of UF/C is configured from one or two or morelayers. In at least one layer of the catalyst coating of UF/C, anaverage thickness of the coating layer is in the range from 25 μm to 160μm, a porosity measured by a weight-in-water method is in the range from50 to 80% by volume, and high-aspect-ratio pores having an aspect ratioof 5 or more account for 0.5 to 50% by volume of the whole volume ofvoids. The high-aspect-ratio pore has an equivalent circle diameter offrom 2 to 50 μm in a cross-sectional image of a catalyst coating layercross section perpendicular to an exhaust gas flow direction and has anaverage aspect ratio of from 10 to 50.

(Substrate)

In the exhaust gas purification catalyst of the present invention, aknown substrate having a honeycomb shape can be used as the substrate ofUF/C, and a honeycomb-shaped monolith substrate (honeycomb filter,high-density honeycomb or the like) or the like is specifically suitablyadopted. The material of such a substrate is also not particularlylimited, and a substrate made of ceramics such as cordierite, siliconcarbide, silica, alumina, and mullite, or a substrate made of a metalsuch as stainless steel including chromium and aluminum is suitablyadopted. Among them, cordierite is preferable in terms of cost.

(Catalyst Coating Layer)

In the exhaust gas purification catalyst of the present invention, thecatalyst coating layer of UF/C is formed on a surface of the substrate,and is configured from one layer or two or more layers, namely, onelayer, two layers, three layers, or four or more layers. Furthermore,the catalyst coating layer of UF/C may not be necessarily uniform overthe entire substrate of the exhaust gas purification catalyst, and mayhave a different composition with respect to each section of thesubstrate, for example, with respect to each of an upstream zone and adownstream zone in an exhaust gas flow direction. Herein, the differentcomposition means that each component forming a catalyst particledescribed below is different, for example.

In the exhaust gas purification catalyst of the present invention, thecatalyst coating layer of UF/C preferably has a two-layered structure.When the catalyst coating layer of UF/C is configured from two or morelayers, each catalyst coating layer of UF/C may have the samecomposition as or a different composition from that of an adjacentcatalyst coating layer. When the catalyst coating layer of UF/C isconfigured from two or more layers, the two or more layers of thecatalyst coating layer can be divided into the uppermost catalystcoating layer, and the lower catalyst coating layer(s) that is presentlower with respect to the uppermost catalyst coating layer.

In the exhaust gas purification catalyst of the present invention, thecatalyst coating layer of UF/C has a structure where a large number ofvoids are included as described below. When the catalyst coating layerof UF/C is configured from two or more layers, at least one layer of thecatalyst coating of UF/C has a structure where a large number of voidsare included as described below. For example, when the catalyst coatinglayer of UF/C is configured from three layers, only one layer serving asthe uppermost layer, only one layer serving as the intermediate layer,or only one layer serving as the undermost layer may have a structurewhere a large number of voids are included, or two or three layersthereof may each have a structure where a large number of voids areincluded. Herein, the “catalyst coating of UF/C” mentioned herein belowconveniently means, unless particularly noted, any catalyst coatingincluding one or two or more layers having a structure where a largenumber of voids are included, as described below.

The catalyst coating layer of UF/C includes a catalyst particle formedfrom a noble metal serving as a main catalyst, a metal oxide, and thelike. Specific examples of the metal oxide forming the catalyst particleinclude aluminum oxide (Al₂O₃, alumina), cerium oxide (CeO₂, ceria),zirconium oxide (ZrO₂, zirconia), silicon oxide (SiO₂, silica), yttriumoxide (Y₂O₃, yttria) and neodymium oxide (Nd₂O₃), as well as compositeoxides thereof. Such metal oxides may be used in combinations of two ormore.

Specific examples of the noble metal forming the catalyst particleinclude platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver(Ag), iridium (Ir) and ruthenium (Ru). Among them, at least one selectedfrom the group consisting of Pt, Rh, Pd, Ir and Ru is preferable, and atleast one selected from the group consisting of Pt, Rh and Pd isparticularly preferable, in terms of catalyst performance. When thecatalyst coating layer of UF/C is configured from two or more layers,one noble metal be preferably used per catalyst coating layer.

The noble metal is preferably supported on the metal oxide describedabove. The amount of the noble metal to be supported is not particularlylimited, and an appropriate amount thereof may be supported depending onthe intended design and the like. The content of the noble metal ispreferably 0.01 to 10 parts by mass, more preferably 0.01 to 5 parts bymass, in terms of metal, based on 100 parts by mass of the catalystparticle. While too small an amount of the noble metal supported tendsto result in an insufficient catalyst activity, and on the other hand,too large an amount thereof tends to cause saturation of catalystactivity and an increase in cost. Any amount in the above preferredrange does not cause such problems.

When the catalyst coating layer of UF/C is configured from two or morelayers, the uppermost catalyst coating layer of two or more of thecatalyst coating layers formed on the substrate is highly exposed tohigh stress, namely, high-temperature and high-concentration gas, andpoisonous substances such as sulfur (S) and hydrocarbon (HC), ascompared with the lower catalyst coating layer(s). Therefore, in orderto provide a catalyst having durability thereto, a noble metal whosecatalyst activity is hardly impaired by stress or poisonous substancesis preferably used in the uppermost catalyst coating layer. Examples ofsuch a noble metal include Rh. In addition, examples of a noble metalwhich is relatively poor resistance to stress and poisonous substancesand preferably used in any other than the uppermost layer catalystcoating include Pd and Pt.

The amount of coating of one layer of the catalyst coating layer of UF/Cis preferably in the range from 50 to 300 g per liter of the volume ofthe substrate. Too small an amount of coating does not impart sufficientcatalyst activity performance of the catalyst particle and thus does notimpart sufficient catalyst performance such as NOx purificationperformance. On the other hand, too large an amount thereof alsoincreases pressure loss to cause fuel efficiency to be deteriorated. Anyamount in the above preferred range does not cause such problems.Herein, the amount of coating of the catalyst coating layer is morepreferably in the range from 50 to 250 g, particularly from 50 to 200 g,per liter of the volume of the substrate, in terms of a balance amongpressure loss, catalyst performance and durability.

The thickness of one layer of the catalyst coating layer of UF/C ispreferably in the range from 25 μm to 160 μm as the average thickness.Too thin a catalyst coating layer does not impart sufficient catalystperformance. On the other hand, too thick a catalyst coating layerincreases the pressure loss in passing of exhaust gas and the like tofail to impart sufficient catalyst performance such as NOx purificationperformance. Any thickness in the above preferred range does not causesuch problems. Herein, the thickness is more preferably in the rangefrom 30 to 96 μm, particularly from 32 to 92 μm, in terms of a balanceamong pressure loss, catalyst performance and durability. The“thickness” of the catalyst coating layer used herein means a length ofthe catalyst coating layer in a direction perpendicular to the center ofa flat portion of the substrate, namely, the shortest distance betweenthe surface of the catalyst coating layer and the surface of thesubstrate (an interface with the lower catalyst coating layer when thelower catalyst coating layer is present). The average thickness of thecatalyst coating layer can be determined by, for example, observing thecatalyst coating layer with a scanning electron microscope (SEM) or anoptical microscope to measure the thickness at each of any 10 points ormore, and calculating the average thickness.

A 15% cumulative size (D15), in a cumulative particle size distributionon a cross-sectional area basis, of the catalyst particle contained inthe catalyst coating layer is preferably 3 to 10 μm, at least withrespect to the catalyst coating of UF/C. Too small a size of thecatalyst particle causes a low porosity and a low gas diffusivity andthus does not impart sufficient catalyst performance such as NOxpurification performance. On the other hand, too large a size thereofcauses a high gas diffusion resistance in the catalyst coating layer andthus does not impart sufficient catalyst performance such as NOxpurification performance. Any particle size in the above preferred rangedoes not cause such problems. Herein, the 15% cumulative size, in acumulative particle size distribution on a cross-sectional area basis,is more preferably in the range from 3 to 9 μm, particularly 3 to 7 μm,in terms of a balance with gas diffusion resistance in the catalystcoating layer and ensuring of coatability with a slurry.

The 15% cumulative size (D15) of the catalyst particle can be determinedby, for example, SEM observation of the cross section of the catalystcoating layer. Specifically, an exemplary procedure is as follows: theexhaust gas purification catalyst is embedded with an epoxy resin or thelike; SEM observation (magnification: 700 to 1500-fold, pixelresolution: 0.2 μm/pixel or more) of a cross-section cut in a radialdirection of the substrate is performed; and the 15% cumulative size, ina cumulative particle size distribution on a cross-sectional area basis,of the catalyst particle is calculated. Herein, the 15% cumulative sizeof the catalyst particle (hereinafter, sometimes designated as “D15”)means a particle size of the catalyst particle which corresponds to theparticle size at 15% in terms of frequency (a cumulative frequency of15% on an area basis) relative to the whole of the cross-sectional areaof the catalyst coating layer when the catalyst particle size(cross-sectional area) is cumulated from the largest cross-sectionalarea of the catalyst particle in the descending order, provided that anypore where the sum of the cross-sectional area of the catalyst particleis less than 0.3 μm² is excluded for the purpose of distinguishing fromnoise. Such observation is preferably performed on a square region of200 μm or more in a horizontal direction to a substrate flat portion and25 μm or more in a perpendicular direction to the substrate flatportion, of the catalyst coating layer. Herein, the particle size refersto a diameter of a minimum circumscribed circle if the cross section isnot circular.

While the catalyst coating layer is formed mainly from the catalystparticle, the catalyst coating layer may also further comprise othercomponent as long as the effect of the present invention is notimpaired. Examples of such other component include other metal oxide andan additive for use in a catalyst coating layer in such a kind of use,and specific examples include one or more of alkali metals such aspotassium (K), sodium (Na), lithium (Li) and cesium (Cs), alkaline earthmetals such as barium (Ba), calcium (Ca) and strontium (Sr), rare-earthelements such as lanthanum (La), yttrium (Y) and cerium (Ce), andtransition metals such as iron (Fe).

It is preferable that the catalyst coating of UF/C be configured fromone or two or more layers, and that at least one layer thereof include alarge number of voids wherein a porosity, as measured by aweight-in-water method, is in the range from 50 to 80% by volume. Toolow a porosity of the catalyst coating of UF/C deteriorates gasdiffusivity and thus does not impart sufficient catalyst performance. Onthe other hand, too high a porosity increases diffusivity to therebyincrease a proportion of gas passing through the coating layer withoutcoming in contact with a catalytic active site, not imparting sufficientcatalyst performance. Any porosity in the above preferred range does notcause such problems. The porosity of a layer of the catalyst coating ofUF/C, the layer having a large number of voids, is more preferably inthe range from 50.9 to 78.8% by volume, particularly preferably in therange from 54 to 78.0% by volume, in terms of a balance between gasdiffusivity and catalyst performance.

The “void(s)” in the catalyst coating of UF/C means a space in thecatalyst coating layer. The shape of the “void” is not particularlylimited, and for example, may be any of spherical, elliptical,cylindrical, cuboid (rectangular column), disc, through-hole shapes, andshapes similar thereto. Such a void encompasses pores such as amicropore having an equivalent circle diameter of a cross-section, ofless than 2 μm; a high-aspect-ratio pore having an equivalent circlediameter of a cross-section, of 2 μm or more, and having an aspect ratioof 5 or more; and a pore having an equivalent circle diameter of across-section, of 2 μm or more, and not having an aspect ratio of 5 ormore. The porosity of the catalyst coating of UF/C can be determined by,for example, subjecting UF/C with a catalyst coating to measurement by aweight-in-water method. Specifically, the porosity can be measured by,for example, a method according to a method prescribed in JIS R 2205.

In the exhaust gas purification catalyst of the present invention,high-aspect-ratio pores having an aspect ratio of 5 or more account for0.5 to 50% by volume of the whole volume of voids in a layer of thecatalyst coating of UF/C, the layer having a large number of voids. Thehigh-aspect-ratio pore is characterized by having an equivalent circlediameter of from 2 to 50 μm in a cross-sectional image of a catalystcoating layer cross section perpendicular to an exhaust gas flowdirection, and an average aspect ratio of from 10 to 50. Accordingly, apore having an equivalent circle diameter of less than 2 μm is notconsidered to be a high-aspect-ratio pore, even if having an aspectratio of 5 or more.

Too low an average aspect ratio of the high-aspect-ratio pore does notimpart sufficient pore connectivity. On the other hand, too high anaverage aspect ratio thereof causes too high a gas diffusivity and thusincreases a proportion of gas passing through the coating layer withoutcoming into contact with a catalytic active site, not impartingsufficient catalyst performance. Any average aspect ratio in the rangefrom 10 to 50 does not cause such problems. The average aspect ratio ofthe high-aspect-ratio pore is more preferably in the range from 10 to35, particularly in the range from 10 to 30, in view of compatibility ofgas diffusivity with catalyst performance.

The average aspect ratio of the high-aspect-ratio pore in the catalystcoating of UF/C can be measured by analyzing a cross-sectional image ofa catalyst coating layer cross section perpendicular to an exhaust gasflow direction (axial direction of a honeycomb-shaped substrate) of thesubstrate, from the three-dimensional information on the pore of thecatalyst coating layer, obtained by FIB-SEM (Focused Ion Beam-ScanningElectron Microscope), X-ray CT, or the like.

Specifically, for example, in the case of FIB-SEM analysis, first, acontinuous cross-sectional image (SEM image) of a catalyst coating layercross section perpendicular to an exhaust gas flow direction of thesubstrate is acquired by FIB-SEM analysis. Next, the resultingcontinuous cross-sectional image is analyzed, and three-dimensionalinformation on a pore having an equivalent circle diameter of across-section, of 2 μm or more, is extracted. FIG. 4 illustrates atwo-dimensional projection diagram exemplifying analysis results ofthree-dimensional information on the pore, obtained by analyzing acontinuous cross-sectional image of a catalyst coating layer crosssection perpendicular to an exhaust gas flow direction of the substrateof the exhaust gas purification catalyst, as one example of analysisresults of three-dimensional information on the pore. As is clear fromthe analysis results of three-dimensional information on the pore shownin FIG. 4, the shape of the pore is indefinite, and a distance forconnecting a starting point and an end point in the continuouscross-sectional image (SEM image) of the pore is defined as“longitudinal size”. Herein, the starting point and the end pointcorrespond to centroids in each SEM image. Next, a constriction portionin a path for connecting the starting point and the end point at theshortest distance in the continuous cross-sectional image (SEM image) ofthe pore is defined. The minimum part whose equivalent circle diameteris 2 μm or more and is also minimum among the constriction portions inthe cross-sectional SEM image is defined as a “throat-shaped portion,”and the equivalent circle diameter thereof in the cross-sectional SEMimage is defined as a “throat-shaped portion size”. (while a pluralityof constriction portions may be present in a pore, the throat-shapedportion size for calculating the aspect ratio is defined as follows: theminimum constriction portion is selected in the path for connecting thestarting point and the end point at the shortest distance, and theequivalent circle diameter of the pore in the cross-sectional SEM imageof the minimum constriction portion (throat-shaped portion) is definedas the “throat-shaped portion size”) Furthermore, the aspect ratio ofthe pore is defined as a “longitudinal size/throat-shaped portion size”.

Next, FIG. 5 illustrates cross-sectional images (SEM images) of (A)(starting point of pore), (B) (throat-shaped portion of pore), (C)(medium point of longitudinal size of pore), (D) (maximum diameterportion having maximum equivalent circle diameter of pore), and (E) (endpoint of pore) in FIG. 4. FIG. 5 is a schematic diagram of across-sectional image (SEM image) of the pore in the catalyst coatinglayer cross section in (A) to (E) of FIG. 4. FIG. 5(A) is a schematicdiagram of a cross-sectional image of the pore at the starting point(one end portion where the equivalent circle diameter of the pore is 2μm or more) in the two-dimensional projection diagram of the poreillustrated in FIG. 4, and G1 represents centroid of the pore in thecross-sectional image. FIG. 5(B) is a schematic diagram of thecross-sectional image of the pore in the throat-shaped portion (whichhas an equivalent circle diameter of the pore of 2 μm or more and is theminimum constriction portion in the path for connecting the startingpoint and the end point at the shortest distance) in the two-dimensionalprojection diagram of the pore illustrated in FIG. 4. FIG. 5(C) is aschematic diagram of the cross-sectional image of the pore at the mediumpoint in the path for connecting the starting point and the end point ofthe longitudinal size at the shortest distance in the two-dimensionalprojection diagram of the pore illustrated in FIG. 4. FIG. 5(D) is across-sectional image of the pore at a position where the equivalentcircle diameter of the pore is maximum in the path for connecting thestarting point and the end point of the longitudinal size at theshortest distance in the two-dimensional projection diagram of the poreillustrated in FIG. 4. FIG. 5(E) is a schematic diagram of across-sectional image of the pore at the end point (other end portionwhere the equivalent circle diameter of the pore is 2 μm or more) in thetwo-dimensional projection diagram of the pore illustrated in FIG. 4,and G2 represents centroid of the pore in the cross-sectional image.Here, the linear distance for connecting the starting point (G1 in FIG.5(A)) of the pore and the end point (G2 in FIG. 5(E)) of the pore inFIG. 5 is defined as the “longitudinal size”. In addition, a portionwhere the equivalent circle diameter in the cross-sectional SEM image is2 μm or more and is minimum, among the constriction portions in the pathfor connecting the starting point and the end point of the pore at theshortest distance, is defined as a “throat-shaped portion”, and theequivalent circle diameter thereof in the cross-sectional SEM image isdefined as a “throat-shaped portion size”. The aspect ratio of the poreis defined as a “longitudinal size/throat-shaped portion size”.Furthermore, the “average aspect ratio of the high-aspect-ratio pore inthe catalyst coating layer” can be determined as follows: aspect ratiosof pores are determined in an area of 500 μm or more in the horizontaldirection to the substrate flat portion, 25 μm or more in theperpendicular direction to the substrate flat portion and 1000 μm ormore in the axial direction to the substrate flat portion, of thecatalyst coating layer, or any area corresponding thereto; and theaverage aspect ratio of the high-aspect-ratio pore having an aspectratio of 5 or more among the pores determined is calculated.

As described above, the rate of the high-aspect-ratio pores relative tothe whole volume of voids in the catalyst coating of UF/C is in therange from 0.5 to 50% by volume. Too low a rate thereof causes poor poreconnectivity. On the other hand, too high a rate thereof causesinsufficient gas diffusivity in a direction perpendicular to an exhaustgas flow direction, not imparting sufficient catalyst performance andalso causing peeling or the like due to reduction in strength of thecatalyst coating layer. Any rate in the above range does not cause suchproblems. Herein, the rate of the high-aspect-ratio pore relative to thewhole volume of voids is preferably in the range from 0.6 to 40.9% byvolume, particularly in the range from 1 to 31% by volume, in terms of abalance among gas diffusivity, catalyst performance, and strength of thecatalyst coating layer.

The rate of the high-aspect-ratio pore relative to the whole volume ofvoids in the catalyst coating of UF/C can be determined by dividing theporosity of the high-aspect-ratio pore (in an area of 500 μm or more inthe horizontal direction to the substrate flat portion, 25 μm or more inthe perpendicular direction to the substrate flat portion, and 1000 μmor more in the axial direction to the substrate flat portion, of thecatalyst coating layer, or any area corresponding thereto) by theporosity of the catalyst coating layer as measured by a weight-in-watermethod.

Furthermore, in the catalyst coating of UF/C, the high-aspect-ratio poreis preferably oriented such that an 80% cumulative angle, in acumulative angle distribution on an angle basis, of an angle (coneangle) between a vector in a longitudinal direction of thehigh-aspect-ratio pore and a vector in an exhaust gas flow direction ofthe substrate is in the range from 0 to 45 degrees. Thus, the gasdiffusivity in an exhaust gas flow direction (axial direction of ahoneycomb-shaped substrate) can be particularly enhanced to therebyenhance the efficiency of utilization of an active site. Too large an80% cumulative angle tends to cause an insufficient component in theaxial direction of the gas diffusivity, reducing the efficiency ofutilization of an active site. Any angle in the above preferred rangedoes not cause such problems. Herein, the 80% cumulative angle ispreferably in the range from 15 to 45 degrees, particularly in the rangefrom 30 to 45 degrees, in terms of catalyst performance.

The cone angle (orientation angle) of the high-aspect-ratio pore in thecatalyst coating of UF/C can be measured by analyzing thecross-sectional image of a catalyst coating layer cross sectionperpendicular to an exhaust gas flow direction (axial direction of ahoneycomb-shaped substrate) of the substrate from the three-dimensionalinformation on the pore of the catalyst coating layer. Specifically, forexample, in the case of FIB-SEM analysis, the “cone angle” can bedetermined from an angle between a vector in a longitudinal directionresulting from the “longitudinal size” of the high-aspect-ratio poreobtained as above and a vector in an exhaust gas flow direction of thesubstrate. FIG. 6 is a schematic diagram illustrating a cone angle(orientation angle) of the high-aspect-ratio pore, and also illustratingone example of a method for determining the “cone angle”. FIG. 6illustrates a vector (Y) in a longitudinal direction of thehigh-aspect-ratio pore and a vector (X) in an exhaust gas flow directionof the substrate in the two-dimensional projection diagram in FIG. 4,and an angle between the vector (Y) in the longitudinal direction andthe vector (X) in an exhaust gas flow direction of the substrate isdefined as the “cone angle”. The three-dimensional information on thepore (three-dimensional image) can be subjected to image analysis, tothereby calculate the 80% cumulative angle, in a cumulative angledistribution on an angle basis, of the cone angle. Herein, the 80%cumulative angle, in a cumulative angle distribution on an angle basis,of the cone angle of the high-aspect-ratio pore means a cone angle ofthe aspect-ratio pore which corresponds to the cone angle at 80% interms of frequency (a cumulative frequency of 80%, on an angle basis ofthe cone angle) relative to the total number of the high-aspect-ratiopores when the number of the high-aspect-ratio pores is counted from thehigh-aspect-ratio pore having the smallest cone angle (degrees) in theascending order. Herein, the 80% cumulative angle, in a cumulative angledistribution on an angle basis, of the cone angle of thehigh-aspect-ratio pore can be determined by randomly extracting 20 ormore of the high-aspect-ratio pores, and determining the 80% cumulativeangle, in a cumulative angle distribution on an angle basis, of the coneangle of each of the high-aspect-ratio pores to provide an averagevalue.

(Method for Producing UF/C)

The method for producing UF/C of the exhaust gas purification catalystcomprising a dual catalyst of a combination of S/C and UF/C of thepresent invention comprises forming the catalyst coating of UF/Cincluding a large number of voids using a catalyst slurry, wherein thecatalyst slurry comprises a noble metal particle having catalystactivity, a metal oxide particle having a 50% cumulative size of 3 to 10μm in a cumulative particle size distribution on a volume basis, and afibrous organic substance in an amount of 0.5 to 9.0 parts by mass basedon 100 parts by mass of the metal oxide particle. The fibrous organicsubstance has an average fiber diameter in a range from 1.7 to 8.0 μmand an average aspect ratio in a range from 9 to 40. When coating thesubstrate with the catalyst slurry and then heating the catalyst slurry,at least a part of the fibrous organic substance is preferably removedto form a void in the catalyst coating layer. Herein, when the catalystcoating layer of UF/C is configured from two or more layers, anycatalyst coating other than the catalyst coating having a structurewhere a large number of voids are included, in the catalyst coatinglayer of UF/C, can be formed by a conventionally known method, forexample, using the same catalyst slurry as described above except forcontaining no fibrous organic substance.

(Metal Oxide Particle)

The metal oxide particle for use in the catalyst production method ofthe present invention has a 50% cumulative size of 3 to 10 μm in acumulative particle size distribution on a volume basis (D50). The 50%cumulative size is preferably a 50% cumulative size, in a cumulativeparticle size distribution on a volume basis, measured by a laserdiffraction method. The metal oxide is the same as described above withrespect to the catalyst particle contained in the catalyst coating layerof the exhaust gas purification catalyst of the present invention. Thepreparation method of the metal oxide particle is not particularlylimited, and a known method can be appropriately adopted. As such ametal oxide particle, a commercially available product may also be used.Examples of the metal oxide particle for use in the method of thepresent invention include a metal oxide particle (including a compositeoxide particle) prepared by a known method, a commercially availablemetal oxide particle (including a composite oxide particle) or a mixturethereof, or a dispersion liquid obtained by dispersing such a particlein a solvent such as ion-exchange water.

Too small a particle size of the metal oxide particle causes too small aparticle size of the catalyst particle (15% cumulative size oncross-sectional area basis) of the catalyst coating layer of theresulting exhaust gas purification catalyst, which results in a decreasein porosity of the catalyst coating layer and therefore deterioration ingas diffusivity to thereby fail to impart sufficient catalystperformance such as NOx purification performance. On the other hand, toolarge a particle size of the metal oxide particle causes too large aparticle size of the catalyst particle (15% cumulative size oncross-sectional area basis) of the catalyst coating layer of theresulting exhaust gas purification catalyst, which results in anincrease in gas diffusion resistance in the catalyst particle to therebyfail to impart sufficient catalyst performance such as NOx purificationperformance. When the 50% cumulative size in a cumulative particle sizedistribution on a volume basis of the metal oxide particle is in therange from 3 to 10 μm, however, such problems are not caused. Herein,the particle size of the metal oxide particle is preferably in the rangefrom 3 to 9 μm, particularly in the range from 3 to 7 μm, in terms of a50% cumulative size on a volume basis in view of a balance amongcoatability, diffraction resistance in the catalyst particle, andcatalyst performance.

The particle size (50% cumulative size on volume basis) of the metaloxide particle can be measured by a laser diffraction method, asdescribed above. Specifically, for example, measurement is conducted on(any) 1000 or more of the metal oxide particles randomly extracted,according to a laser diffraction method with a laser diffractionapparatus such as a laser diffraction particle size distributionmeasurement apparatus, and the 50% cumulative size, in a cumulativeparticle size distribution on a volume basis, of the metal oxideparticle is calculated. Herein, the 50% cumulative size on a volumebasis of the metal oxide particle means a particle size of the metaloxide particle which corresponds to the particle size at 50% in terms offrequency (a cumulative frequency of 50% on a volume basis) relative tothe total number of the metal oxide particles when the number of themetal oxide particles is counted from the smallest metal oxide particlesize (area) in the ascending order. Herein, the particle size refers toa diameter of a minimum circumscribed circle when the cross section isnot circular.

The method for preparing the metal oxide particle having such a particlesize is not particularly limited, and is as follows, for example: a rawmaterial of the metal oxide particle, such as a metal oxide particlepowder, is first provided; the metal oxide particle powder or the likeis then mixed with a solvent such as ion-exchange water; and thereafterthe resulting solution is subjected to stirring and dispersing of themetal oxide particle powder or the like in a solvent such as water usinga medium mill such as a bead mill, other stirring type pulverizingapparatus, or the like to adjust the particle size of the metal oxideparticle. Herein, stirring conditions in the case of using a medium millsuch as a bead mill are not particularly limited, and the bead size, thetreatment time, and the stirring speed are preferably in the range from100 to 5000 μm, 3 minutes to 1 hour, and 50 to 500 rpm, respectively.

(Preparation and Application of Catalyst Slurry)

In the method for producing UF/C of the exhaust gas purificationcatalyst of the present invention, a catalyst slurry is used, thecatalyst slurry comprising a noble metal particle having catalystactivity, a metal oxide particle having the 50% cumulative size of 3 to10 μm in a cumulative particle size distribution on a volume basis, anda fibrous organic substance in an amount of 0.5 to 9.0 parts by massbased on 100 parts by mass of the metal oxide particle.

The noble metal raw material for preparation of the noble metal particleis not particularly limited, and examples thereof include a solutionobtained by dissolving a salt (for example, acetate, carbonate, nitrate,an ammonium salt, citrate, or a dinitrodiammine salt) of a noble metal(for example, Pt, Rh, Pd or Ru, or a compound thereof), or a complexthereof (for example, a tetraammine complex) in a solvent such as wateror alcohol. In addition, the amount of the noble metal is notparticularly limited, the noble metal may be appropriately supported ina required amount depending on the intended design and the like, and theamount is preferably 0.01% by mass or more. Herein, when platinum isused as the noble metal, a platinum salt is not particularly limited,and examples thereof include acetate, carbonate, nitrate, an ammoniumsalt, citrate or a dinitrodiammine salt of platinum (Pt), or a complexthereof. Among them, a dinitrodiammine salt is preferable because it iseasily supported and has a high dispersibility. When palladium is usedas the noble metal, a palladium salt is not particularly limited, andexamples thereof include a solution of acetate, carbonate, nitrate, anammonium salt, citrate, a dinitrodiammine salt of palladium (Pd), or acomplex thereof. Among them, nitrate or a dinitrodiammine salt ispreferable because it is easily supported and has a high dispersibility.Furthermore, the solvent is not particularly limited, and examplesthereof include a solvent that can allow dissolution in the form of anion, such as water (preferably pure water such as ion-exchange water anddistilled water).

The fibrous organic substance is not particularly limited as long as itis a substance that can be removed by a heating step described below,and examples thereof include a polyethylene terephthalate (PET) fiber,an acrylic fiber, a nylon fiber, a rayon fiber, and a cellulose fiber.Among them, at least one selected from the group consisting of a PETfiber and a nylon fiber is preferably used in terms of a balance betweenprocessability and the firing temperature. By using a catalyst slurrycontaining such a fibrous organic substance and at least partiallyremoving the fibrous organic substance in a subsequent step, a voidhaving the same shape as that of the fibrous organic substance can beformed in the catalyst coating layer. The void thus formed can serve asa diffusion path of exhaust gas and the resulting catalyst can exhibitexcellent catalyst performance even in a region under a high load with ahigh flow rate of gas.

The fibrous organic substance for use in the catalyst production methodof the present invention has an average fiber diameter ranging from 1.7to 8.0 μm. Too small an average fiber diameter does not impart aneffective high-aspect-ratio pore, resulting in insufficient catalystperformance. On the other hand, too large an average fiber diameterincreases the thickness of the catalyst coating layer, therebyincreasing pressure loss to cause deterioration in fuel efficiency. Anyaverage fiber diameter in the above range does not cause such problems.The average fiber diameter of the fibrous organic substance ispreferably in the range from 2.0 to 6.0 μm, particularly in the rangefrom 2.0 to 5.0 μm, in terms of a balance between catalyst performanceand coating thickness.

The fibrous organic substance for use in the catalyst production methodof the present invention has an average aspect ratio in the range from 9to 40. Too low an average aspect ratio results in insufficient poreconnectivity to thereby cause gas diffusivity to be insufficient. On theother hand, too high an average aspect ratio causes too high adiffusivity to thereby increase a proportion of gas passing through thecoating layer without coming into contact with a catalytic active site,not imparting sufficient catalyst performance. Any average aspect ratioin the above range does not cause such problems. The average aspectratio of the fibrous organic substance is preferably in the range from 9to 30, particularly in the range from 9 to 28, in terms of a balancebetween gas diffusivity and catalyst performance. Herein, the averageaspect ratio of the fibrous organic substance is defined as an “averagefiber length/average fiber diameter”. Herein, the fiber length means thelinear distance for connecting the starting point and the end point ofthe fiber. The average fiber length can be determined by randomlyextracting 50 or more of the fibrous organic substances, measuring thefiber length of each of the fibrous organic substances, and calculatingan average value. In addition, the average fiber diameter can bedetermined by randomly extracting 50 or more of the fibrous organicsubstances, measuring the fiber diameter of each of the fibrous organicsubstances, and calculating an average value.

In the catalyst production method of the present invention, the fibrousorganic substance is used in an amount of 0.5 to 9.0 parts by mass basedon 100 parts by mass of the metal oxide particle in a catalyst slurryfor formation of the lower catalyst coating layer. Too small an amountof the fibrous organic substance mixed fails to impart sufficient poreconnectivity, resulting in insufficient catalyst performance. On theother hand, too large an amount thereof increases the thickness of thecatalyst coating layer, thereby increasing pressure loss to causedeterioration in fuel efficiency. Any amount in the above range does notcause such problems. Herein, the fibrous organic substance is preferablyused in an amount of 0.5 to 8.0 parts by mass, particularly 1.0 to 5.0parts by mass, based on 100 parts by mass of the metal oxide particle inthe catalyst slurry, in terms of a balance between catalyst performanceand pressure loss. Herein, the fibrous organic substance more preferablyhas an average fiber diameter in the range from 2.0 to 6.0 μm and anaverage aspect ratio in the range from 9 to 30.

The method for preparing the catalyst slurry is not particularlylimited. The metal oxide particle, the noble metal raw material, and thefibrous organic substance may be mixed, if necessary with a known binderor the like, and a known method can be appropriately adopted. Herein,conditions of such mixing are not particularly limited. For example, thestirring speed and the treatment time are preferably in the range from100 to 400 rpm and 30 minutes or more, respectively, and the fibrousorganic substance may be uniformly dispersed and mixed in the catalystslurry. The mixing order is not particularly limited, and there may beadopted any of the following methods, for example: a method where thenoble metal raw material is mixed with a dispersion liquid including themetal oxide particle to support the noble metal, and thereafter thefibrous organic substance is mixed therewith; a method where the fibrousorganic substance is mixed with a dispersion liquid including the metaloxide particle and thereafter the noble metal raw material is mixedtherewith; a method where the noble metal raw material and the fibrousorganic substance are simultaneously mixed in a dispersion liquidincluding the metal oxide particle; and a method where the metal oxideparticle and the fibrous organic substance are mixed with a solutionincluding the noble metal raw material. Treatment conditions are notparticularly limited, and are appropriately selected depending on thedesign of the intended exhaust gas purification catalyst or the like.

The surface of the substrate is coated with the catalyst slurry tothereby form a catalyst slurry layer preferably such that the amount ofcoating and the average thickness per one layer of the catalyst coatinglayer after firing are in a range from 50 to 300 g per liter of thevolume of the substrate and in a range from 25 μm to 160 μm,respectively. The coating method is not particularly limited, and aknown method can be appropriately adopted. Specific examples include amethod where a honeycomb-shaped substrate is dipped in to coat thesubstrate with the catalyst slurry (dipping method), a wash coat method,a method where the catalyst slurry is injected by an injection means,and the like. Herein, the surface of the honeycomb-shaped substrate isneeded to be coated with the catalyst slurry under coating conditionssuch that the following are satisfied: the amount of coating per onelayer of the catalyst coating layer after firing is in the range from 50to 300 g per liter of the volume of the substrate, and the averagethickness per one layer of the catalyst coating layer after firing is inthe range from 25 μm to 160 μm.

In the production method of UF/C of the present invention, the substrateis coated with the catalyst slurry, and then heated to thereby evaporatethe solvent or the like included in the slurry and also remove thefibrous organic substance. Such heating is typically conducted by firingthe substrate coated with the catalyst slurry. Such firing is preferablyconducted at a temperature in the range from 300 to 800° C.,particularly preferably from 400 to 700° C. Too low a firing temperaturetends to cause the fibrous organic substance to remain, and on the otherhand, too high a firing temperature tends to sinter the noble metalparticle. Any firing temperature in the above preferred range does notcause such problems. The firing time varies depending on the firingtemperature, and is preferably 20 minutes or more, more preferably 30minutes to 2 hours. Furthermore, the atmosphere in firing is notparticularly limited, and is preferably in the air or in an atmosphereof inert gas such as nitrogen (N₂).

In the exhaust gas purification catalyst of the present invention, whenUF/C having two or more catalyst coating layers different in compositionfrom each other is produced, it can be prepared by coating the substratewith the catalyst slurry and heating it to thereby form the catalystcoating on the substrate, and coating again the catalyst coating with acatalyst slurry different therefrom in terms of composition, namely, theamounts and the types of the metal oxide, the noble metal and the like,and heating it, in a repeated manner.

In one embodiment of the exhaust gas purification catalyst of thepresent invention, the catalyst coating layer of UF/C is configured fromtwo layers, and can be prepared by using a catalyst slurry including theabove fibrous organic substance, in addition to platinum as a noblemetal particle and a ceria zirconia solid solution and alumina as metaloxide particles, to form the lower catalyst coating layer, andthereafter using a catalyst slurry including the above fibrous organicsubstance, in addition to rhodium as a noble metal particle and zirconiaand alumina as metal oxide particles, to form the uppermost layercatalyst coating thereon.

(Configuration of S/C)

S/C in the exhaust gas purification catalyst of the present inventioncan be prepared by forming the catalyst coating on the substrateaccording to a conventionally known method. That is, S/C can be preparedin the same manner as in the catalyst coating of UF/C except that nopore-forming material is used in preparation of the catalyst coating information of the catalyst coating of UF/C in the exhaust gaspurification catalyst of the present invention.

The amount of coating per one layer of the catalyst coating layer of S/Cis preferably in the range from 50 to 300 g per liter of the volume ofthe substrate. S/C is disposed on an upstream side over UF/C in anexhaust gas flow direction, and therefore the amount of coating ispreferably larger than the amount of coating of the catalyst coatinglayer of UF/C. Too small an amount of coating cannot provide sufficientcatalyst activity performance of the catalyst particle and thus does notprovide sufficient catalyst performance such as NOx purificationperformance. On the other hand, too large an amount thereof increasespressure loss to cause fuel efficiency to be deteriorated. Any amount inthe above preferred range does not cause such problems. Herein, theamount of coating of the catalyst coating layer per unit volume of thesubstrate is more preferably in the range from 50 to 280 g/L,particularly preferably in the range from 50 to 250 g/L, in terms of abalance among pressure loss, catalyst performance and durability.

In one embodiment of the exhaust gas purification catalyst of thepresent invention, the catalyst coating layer of S/C is configured fromtwo layers, and the upper layer and the lower layer contain Rh and Pd,respectively, as a metal having catalyst activity.

(Embodiment of Usage of Exhaust Gas Purification Catalyst of the PresentInvention)

The exhaust gas purification catalyst of the present invention is anexhaust gas purification catalyst comprising a dual catalyst of acombination of S/C and UF/C that is disposed posterior to the S/C in anexhaust gas flow direction, and is preferably used under a high intakeair mass condition, in particular, during the transient period of anair-fuel ratio. The catalyst coating of the UF/C has the abovecharacteristics.

The high intake air mass condition here is preferably such that theintake air mass Ga is 40 g/s or more.

During the transient period of an air-fuel ratio, the air-fuel ratio A/Fis preferably actively controlled as targets of 14.1 and 15.1.

Herein, the configuration of the exhaust gas purification catalyst ofthe present invention can be applied to, for example, a tandem typeexhaust gas purification catalyst as well as the dual catalyst of acombination of S/C and UF/C.

In one embodiment of the exhaust gas purification catalyst of thepresent invention, the same catalyst coating as the catalyst coating ofUF/C described above can be applied to a catalyst coating at a rearstage in an exhaust gas flow direction in a tandem type exhaust gaspurification catalyst.

The catalyst coating at a rear stage has high-aspect-ratio poressatisfying predetermined conditions to thereby especially enhance thegas diffusivity of the catalyst coating at a rear stage, therebyenabling the tandem type exhaust gas purification catalyst to exhibitsufficient purification performance under a high Ga condition during thetransient period of A/F, as in the dual catalyst of a combination of S/Cand UF/C.

The exhaust gas purification catalyst of the present invention is usedfor a method for purifying exhaust gas where exhaust gas discharged froman internal combustion engine is brought into contact with the catalyst.The method for bringing exhaust gas into contact with the exhaust gaspurification catalyst is not particularly limited, and a known methodcan be appropriately adopted. For example, a method may be adopted wherethe exhaust gas purification catalyst according to the present inventionis disposed in an exhaust gas tube through which gas discharged from aninternal combustion engine flows, thereby bringing exhaust gasdischarged from an internal combustion engine into contact with theexhaust gas purification catalyst.

The exhaust gas purification catalyst of the present invention exhibitsexcellent catalyst performance even in a region under a high load with ahigh flow rate of gas. Therefore, for example, when exhaust gasdischarged from an internal combustion engine of an automotive or thelike is brought into contact with the exhaust gas purification catalystof the present invention, exhaust gas can be purified even in a regionunder a high load with a high flow rate of gas. The exhaust gaspurification catalyst of the present invention can be used for purifyingharmful components such as harmful gases (hydrocarbon (HC), carbonmonoxide (CO), nitrogen oxides (NOx)) in exhaust gas discharged from aninternal combustion engine of an automotive or the like.

(Embodiments of Use of Exhaust Gas Purification Catalyst of PresentInvention)

The exhaust gas purification catalyst of the present invention may beused singly or in combination with other catalyst. Such other catalystis not particularly limited, and a known catalyst (for example, in thecase of an exhaust gas purification catalyst for automotives, anoxidation catalyst, a NOx reduction catalyst, a NOx storage reductioncatalyst (NSR catalyst), a lean NOx trap catalyst (LNT catalyst), a NOxselective reduction catalyst (SCR catalyst), or the like) may beappropriately used.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to Examples, but the present invention is not intended to belimited to these Examples.

Test 1: Preparation and Evaluation of Catalyst Including One CatalystCoating Layer Having Void

1. Preparation of Catalyst

(1) Example 1

First, 150 g of an Al₂O₃ powder (produced by Sasol: specific surfacearea: 100 m²/g, average particle size: 30 μm) and 300 g of a powder of aCeO₂—ZrO₂ solid solution (produced by Daiichi Kigenso Kagaku Kogyo Co.,Ltd.: CeO₂ content: 20% by mass, ZrO₂ content: 25% by mass, specificsurface area: 100 m²/g, average particle size: 10 nm) were added to 500g of ion-exchange water and mixed to provide a solution. The solutionwas subjected to a stirring treatment under the following conditions: abead mill (produced by As One Corporation, trade name “alumina ball”,beads used: microbeads having a diameter of 5000 μm and made of alumina)was used, and the treatment time was 25 minutes and the stirring speedwas 400 rpm. Thus, a dispersion liquid including a metal oxide particlemade of a mixture (composite metal oxide) of the CeO₂—ZrO₂ solidsolution and the Al₂O₃ powder was prepared. Herein, the particle size ofthe metal oxide particle was measured by using a laser diffractionparticle size distribution measurement apparatus (manufactured by HORIBALtd., trade name “LA-920”) according to a laser diffraction method, andit was found that the 50% cumulative size in a cumulative particle sizedistribution on an area basis was 3.2 μm.

Next, 0.05 L of a dinitroammine platinum solution including 4 g ofplatinum (Pt) in terms of metal, as a noble metal raw material, and 1.0part by mass of an organic fiber (PET fiber, average diameter: 3μm×length: 42 μm, average aspect ratio: 14) as a fibrous organicsubstance based on 100 parts by mass of the metal oxide particle wereadded to the resulting dispersion liquid, and the resultant was mixedunder a condition of a stirring speed of 400 rpm for 30 minutes, therebypreparing a catalyst slurry.

Next, a hexagonal cell cordierite monolith honeycomb substrate(manufactured by Denso Corporation, trade name “D60H/3-9R-08EK”,diameter: 103 μm, length: 105 μm, volume: 875 ml, cell density: 600cell/inch²) as a substrate was wash coated with the resulting catalystslurry, and dried in the air under a temperature condition of 100° C.for 0.5 hours. Thereafter wash coating of the substrate with such acatalyst slurry, and drying/calcining were repeatedly performed so thatthe amount of coating on the substrate was 100 g per liter of the volumeof the substrate, thereby forming a catalyst slurry layer on thesubstrate.

Thereafter, the resultant was fired in the air under a temperaturecondition of 500° C. for 2 hours, thereby providing an exhaust gaspurification catalyst (catalyst sample) in which a catalyst coatinglayer made of a catalyst particle was formed on the substrate surfacemade of the honeycomb-shaped cordierite monolith substrate.

Table 1 shows the treatment time [min] of the stirring treatment and theparticle size (50% cumulative size on volume basis) [μm] of theresulting metal oxide particle in the oxide particle preparation step;the type of a raw material, the average fiber diameter [μm], the averageaspect ratio and the amount mixed [parts by mass] of the fibrous organicsubstance used in the catalyst slurry preparation step; and the amountof coating [g/L] of the catalyst coating layer.

TABLE 1 Oxide particle preparation step Fibrous organic substanceStirring Metal oxide particle Average treatment Particle size fiberdiameter Amount Treatment (50% cumulative size or average Average mixedtime on volume basis) Type of diameter aspect (parts (min) [μm] rawmaterial [μm] ratio by mass) Example 1 25 3.2 PET fiber 3.0 14 1.0Example 2 15 4.6 PET fiber 3.0 14 1.0 Example 3 13 6.4 PET fiber 3.0 141.0 Example 4 7 9.5 PET fiber 3.0 14 1.0 Example 5 15 4.6 PET fiber 3.014 2.0 Example 6 17 6.4 PET fiber 3.0 14 2.0 Example 7 5 9.5 PET fiber3.0 14 2.0 Example 8 25 3.2 PET fiber 3.0 14 5.0 Example 9 15 4.6 PETfiber 3.0 14 5.0 Example 10 5 9.5 PET fiber 3.0 14 5.0 Example 11 26 3.2PET fiber 3.0 14 7.0 Example 12 17 4.6 PET fiber 3.0 14 7.0 Example 1312 6.4 PET fiber 3.0 14 7.0 Example 14 4 9.5 PET fiber 3.0 14 7.0Example 15 24 3.2 PET fiber 2.0 21 3.0 Example 16 14 4.6 PET fiber 2.021 3.0 Example 17 11 6.4 PET fiber 2.0 21 3.0 Example 18 27 3.2 PETfiber 2.0 21 5.0 Example 19 15 4.6 PET fiber 2.0 21 5.0 Example 20 126.4 PET fiber 2.0 21 5.0 Example 21 24 3.2 PET fiber 5.0 10 1.0 Example22 19 4.6 PET fiber 5.0 10 1.0 Example 23 13 6.4 PET fiber 5.0 10 1.0Catalyst coating layer Particle size of High-aspect-ratio pore catalystparticle Rate Catalyst (15% cumulative relative performance size on towhole NOx Amount of cross-sectional Average Porosity Average Orientationof voids conversion coating area basis) thickness (% by aspect angle (%by efficiency [g/L] [μm] [μm] volume) ratio (degrees) volume) [%]Example 1 100 3.4 32.0 51.2 17.6 38.1 2.4 83.1 Example 2 100 5.4 56.266.8 19.1 41.3 7.4 84.6 Example 3 100 6.9 69.0 73.2 19.8 38.6 7.8 82.1Example 4 100 9.7 86.5 78.6 21.0 42.3 8.6 81.7 Example 5 100 5.4 52.764.9 18.9 38.4 11.1 89.1 Example 6 100 6.9 68.2 71.7 20.7 39.5 9.1 87.5Example 7 100 9.7 88.8 77.8 21.4 42.1 15.2 84.8 Example 8 100 3.4 40.459.0 20.0 40.3 49.7 88.7 Example 9 100 5.4 58.8 68.9 20.4 39.3 18.8 90.4Example 10 100 9.7 90.3 78.8 22.1 43.3 20.1 86.1 Example 11 100 3.4 37.556.5 19.9 37.5 34.4 83.7 Example 12 100 5.4 60.6 69.7 21.1 38.7 22.883.9 Example 13 100 6.9 70.0 72.6 21.3 44.4 14.0 82.9 Example 14 10010.0 96.1 79.5 21.6 41.9 23.9 82.5 Example 15 100 3.4 33.0 53.2 22.724.7 14.3 84.2 Example 16 100 5.4 59.0 68.7 23.4 25.0 17.5 82.5 Example17 100 6.9 65.6 70.5 25.5 27.7 3.4 85.9 Example 18 100 3.4 38.0 57.226.2 24.5 39.2 85.9 Example 19 100 5.4 62.6 70.5 25.7 24.6 27.0 84.2Example 20 100 6.9 68.7 72.5 26.9 27.2 13.3 91.1 Example 21 100 3.4 30.550.9 10.5 42.2 0.7 87.8 Example 22 100 5.4 49.5 65.1 11.0 43.2 0.6 86.4Example 23 100 6.9 63.2 71.1 13.4 44.7 6.1 91.5

(2) Examples 2 to 42

Each catalyst slurry was obtained in the same manner as in Example 1except that the treatment time by a bead mill was changed as shown inTable 1 and Table 2, the stirring treatment was performed by a bead millso that the particle size of the metal oxide particle was as shown inTable 1 and Table 2 in terms of the 50% cumulative size in a cumulativeparticle size distribution on a volume basis, and a fibrous organicsubstance, the type of a raw material, the average fiber diameter, theaverage aspect ratio and the amount mixed of which were as shown inTable 1 and Table 2, was used as the fibrous organic substance. Next, acordierite monolith honeycomb substrate was coated with the resultingcatalyst slurry in the same manner as in Example 1, and the resultantwas fired to provide an exhaust gas purification catalyst (catalystsample).

Herein, each fibrous organic substance used in Examples 31 to 39 wasprepared by adding titanium isopropoxide (Ti(OPri)₄), polyethyleneglycol (PEG) and a polymethyl methacrylate resin (PMMA) particle(average diameter: 3 μm) to isopropanol, and pouring the resultant intodistilled water, thereby preparing an organic fiber having apredetermined shape.

Table 1 and Table 2 show the treatment time [min] of the stirringtreatment and the particle size (50% cumulative size on volume basis)[μm] of the resulting metal oxide particle in the oxide particlepreparation step; the type of a raw material, the average fiber diameter[μm], the average aspect ratio and the amount mixed [parts by mass] ofthe fibrous organic substance used in the catalyst slurry preparationstep; and the amount of coating [g/L] with the catalyst coating layer.

TABLE 2 Oxide particle preparation step Fibrous organic substanceStirring Metal oxide particle Average treatment Particle size fiberdiameter Amount Treatment (50% cumulative or average Average mixed timesize on volume diameter aspect (parts (min) basis) [μm] Type of rawmaterial [μm] ratio by mass) Example 24 6 9.5 PET fiber 5.0 10 2.0Example 25 24 3.2 PET fiber 5.0 10 2.0 Example 26 16 4.6 PET fiber 5.010 2.0 Example 27 6 9.5 PET fiber 5.0 10 2.0 Example 28 25 3.2 PET fiber5.0 10 3.0 Example 29 16 4.6 PET fiber 5.0 10 3.0 Example 30 24 3.2 PETfiber 5.0 10 5.0 Example 31 16 4.6 PMMA particle + TiOPr + PEG 3.0 113.0 Example 32 13 6.4 PMMA particle + TiOPr + PEG 3.0 11 3.0 Example 3325 3.2 PMMA particle + TiOPr + PEG 3.0 11 5.0 Example 34 15 4.6 PMMAparticle + TiOPr + PEG 3.0 11 5.0 Example 35 4 9.5 PMMA particle +TiOPr + PEG 3.0 11 5.0 Example 36 25 3.2 PMMA particle + TiOPr + PEG 3.011 7.0 Example 37 17 4.6 PMMA particle + TiOPr + PEG 3.0 11 7.0 Example38 12 6.4 PMMA particle + TiOPr + PEG 3.0 11 7.0 Example 39 5 9.5 PMMAparticle + TiOPr + PEG 3.0 11 7.0 Example 40 16 4.6 PET fiber 2.0 40 3.0Example 41 16 4.6 PET fiber 2.0 40 3.0 Example 42 16 4.6 PET fiber 2.040 3.0 Catalyst coating layer Particle size of High-aspect-ratio porecatalyst particle Rate Catalyst (15% cumulative relative performancesize on to whole NOx Amount of cross-sectional Average Porosity AverageOrientation of voids conversion coating area basis) thickness (% byaspect angle (% by efficiency [g/L] [μm] [μm] volume) ratio (degrees)volume) [%] Example 24 100 9.7 85.0 76.9 14.6 41.3 11.0 84.3 Example 25100 3.4 31.5 52.8 11.0 41.4 12.1 87.8 Example 26 100 6.9 65.2 70.6 13.941.4 3.4 90.7 Example 27 100 9.7 84.7 76.5 16.3 42.1 9.0 84.3 Example 28100 3.4 33.7 53.7 11.7 41.1 17.5 88.5 Example 29 100 5.4 55.5 66.2 12.540.3 4.1 83.7 Example 30 100 3.4 36.3 56.4 12.8 38.3 34.1 88.7 Example31 100 5.4 57.5 68.7 12.1 41.0 17.5 80.9 Example 32 100 6.9 68.8 71.711.1 42.3 9.4 91.3 Example 33 100 3.4 37.2 57.5 13.0 43.7 40.9 85.9Example 34 100 5.4 56.0 66.7 13.8 43.8 7.0 91.8 Example 35 100 9.7 95.578.1 16.5 43.9 28.7 85.9 Example 36 100 3.4 35.1 54.6 17.0 41.1 23.192.9 Example 37 100 5.4 57.8 68.5 17.9 42.0 16.3 95.5 Example 38 100 6.969.2 73.0 20.0 41.1 15.8 96.8 Example 39 100 9.7 90.0 78.7 20.9 40.119.8 91.8 Example 40 50 5.4 28.0 66.9 40.2 19.8 35.9 81.1 Example 41 2005.4 102.3 68.2 43.5 20.0 38.2 82.3 Example 42 300 5.4 152.1 69.4 46.218.6 39.4 81.9

(3) Comparative Examples 1 to 7

Each comparative catalyst slurry was obtained in the same manner as inExample 1 except that the treatment time by a bead mill was changed asshown in Table 3, the stirring treatment was performed by a bead mill sothat the particle size of the metal oxide particle was as shown in Table3 in terms of the 50% cumulative size in a cumulative particle sizedistribution on a volume basis, and no organic substance (fibrousorganic substance) was used. Next, a cordierite monolith honeycombsubstrate was coated with the resulting comparative catalyst slurry inthe same manner as in Example 1, and the resultant was fired to providea comparative exhaust gas purification catalyst (comparative catalystsample).

Table 3 shows the treatment time [min] of the stirring treatment and theparticle size (50% cumulative size on volume basis) [μm] of theresulting metal oxide particle in the oxide particle preparation step,and the amount of coating [g/L] with the catalyst coating layer.

TABLE 3 Oxide particle preparation step Fibrous organic substance ororganic substance Stirring Metal oxide particle Average treatmentParticle size fiber diameter Amount Treatment (50% cumulative or averageAverage mixed time size on volume Type of diameter aspect (parts (min)basis) [μm] raw material [μm] ratio by mass) Comparative Example 1 410.7 — — — — Comparative Example 2 36 1.9 — — — — Comparative Example 324 3.2 — — — — Comparative Example 4 15 4.6 — — — — Comparative Example5 12 6.4 — — — — Comparative Example 6 6 9.5 — — — — Comparative Example7 3 12.0 — — — — Comparative Example 8 14 4.6 PET fiber 3.0 21 1.0Comparative Example 9 3 12.0 PET fiber 3.0 21 1.0 Comparative Example 1045 0.7 PET fiber 3.0 21 3.0 Comparative Example 11 3 12.0 PET fiber 3.021 3.0 Comparative Example 12 45 0.7 PET fiber 3.0 21 5.0 ComparativeExample 13 24 3.2 PET fiber 3.0 21 5.0 Comparative Example 14 45 0.7 PETfiber 3.0 21 7.0 Comparative Example 15 3 12.0 PET fiber 3.0 21 7.0Comparative Example 16 3 12.0 PET fiber 3.0 14 0.5 Comparative Example17 3 12.0 PET fiber 3.0 14 2.0 Comparative Example 18 3 12.0 PET fiber3.0 14 5.0 Comparative Example 19 3 12.0 PET fiber 3.0 14 9.0Comparative Example 20 24 3.2 PET fiber 3.0 50 1.0 Comparative Example21 14 4.6 PET fiber 3.0 50 1.0 Comparative Example 22 17 6.4 PET fiber3.0 50 1.0 Comparative Example 23 6 9.5 PET fiber 3.0 50 1.0 Catalystcoating layer Particle size of High-aspect-ratio pore catalyst particleRate Catalyst (15% cumulative relative performance size on to whole NOxAmount of cross-sectional Average Porosity Average Orientation of voidsconversion coating area basis) thickness (% by aspect angle (% byefficiency [g/L] [μm] [μm] volume) ratio (degrees) volume) [%]Comparative Example 1 100 0.9 18.0 39.7 5.0 74.0 0.01 60.0 ComparativeExample 2 100 2.4 23.0 44.7 5.2 80.0 0.02 65.2 Comparative Example 3 1003.4 28.0 50.3 5.5 63.0 0.04 73.0 Comparative Example 4 100 5.4 52.0 64.85.6 77.0 0.06 73.8 Comparative Example 5 100 6.9 62.0 69.3 5.8 65.0 0.0872.4 Comparative Example 6 100 9.7 84.0 74.1 5.7 66.0 0.09 72.1Comparative Example 7 100 13.0 91.0 79.1 6.0 57.4 0.11 63.5 ComparativeExample 8 100 5.4 53.2 65.0 6.8 61.4 0.10 74.7 Comparative Example 9 10013.0 89.5 77.9 6.5 70.2 0.12 62.6 Comparative Example 10 100 0.9 29.650.9 6.6 59.2 59.86 63.7 Comparative Example 11 100 13.0 94.8 80.0 7.160.2 11.60 67.6 Comparative Example 12 100 0.9 29.3 50.8 7.0 54.1 59.4661.7 Comparative Example 13 100 3.4 40.6 59.6 6.9 60.4 51.19 77.0Comparative Example 14 100 0.9 30.1 50.8 7.5 53.1 59.58 60.3 ComparativeExample 15 100 13.0 91.5 78.6 7.3 60.0 15.32 64.0 Comparative Example 16100 13.0 91.0 79.3 12.1 65.0 5.24 67.3 Comparative Example 17 100 13.093.6 78.9 15.1 56.3 8.39 76.7 Comparative Example 18 100 13.0 97.9 81.416.1 40.3 11.26 76.5 Comparative Example 19 100 13.0 95.8 80.4 17.6 42.115.26 60.3 Comparative Example 20 100 3.4 30.0 50.9 50.2 7.2 55.14 74.3Comparative Example 21 100 5.4 54.4 65.7 50.0 8.2 57.33 72.9 ComparativeExample 22 100 6.9 63.8 71.3 53.0 9.2 59.92 72.1 Comparative Example 23100 9.7 82.7 77.2 50.1 10.1 61.44 72.5

(4) Comparative Example 8 to 133

Each comparative catalyst slurry was obtained in the same manner as inExample 1 except that the treatment time by a bead mill was changed asshown in Table 3 to Table 8, the stirring treatment was performed by abead mill so that the particle size of the metal oxide particle was asshown in Table 3 to Table 8 in terms of the 50% cumulative size in acumulative particle size distribution on a volume basis, a fibrousorganic substance or an organic substance, the type of a raw material,the average fiber diameter or the average diameter, the average aspectratio and the amount mixed of which were as shown in Table 3 to Table 8,was used as the fibrous organic substance or the organic substance.Next, a cordierite monolith honeycomb substrate was coated with theresulting comparative catalyst slurry in the same manner as in Example1, and the resultant was fired to provide a comparative exhaust gaspurification catalyst (comparative catalyst sample).

Herein, each organic substance (fibrous organic substance) used inComparative Examples 127 to 131 was prepared by adding titaniumisopropoxide (Ti(OPri)₄), polyethylene glycol (PEG) and a polymethylmethacrylate resin (PMMA) particle (average diameter: 3 μm) toisopropanol, and pouring the resultant into distilled water, therebypreparing an organic fiber having a predetermined shape.

Table 3 to Table 8 show the treatment time [min] of the stirringtreatment and the particle size (50% cumulative size on volume basis)[μm] of the resulting metal oxide particle in the oxide particlepreparation step, the type of a raw material, the average fiber diameteror the average diameter [μm], the average aspect ratio and the amountmixed [parts by mass] of the fibrous organic substance or the organicsubstance used in the catalyst slurry preparation step, and the amountof coating [g/L] with the catalyst coating layer.

TABLE 4 Oxide particle preparation step Fibrous organic substance ororganic substance Stirring Metal oxide particle Average treatmentParticle size fiber diameter Amount Treatment (50% cumulative or averageAverage mixed time size on volume Type of diameter aspect (parts (min)basis) [μm] raw material [μm] ratio by mass) Comparative Example 24 312.0 PET fiber 3.0 50 1.0 Comparative Example 25 12 6.4 PET fiber 3.0 502.0 Comparative Example 26 3 12.0 PET fiber 3.0 50 2.0 ComparativeExample 27 24 3.2 PET fiber 3.0 50 3.0 Comparative Example 28 12 6.4 PETfiber 3.0 50 3.0 Comparative Example 29 5 9.5 PET fiber 3.0 50 3.0Comparative Example 30 24 3.2 PET fiber 3.0 50 5.0 Comparative Example31 14 4.6 PET fiber 3.0 50 5.0 Comparative Example 32 3 12.0 PET fiber3.0 50 5.0 Comparative Example 33 24 3.2 PET fiber 3.0 50 7.0Comparative Example 34 14 4.6 PET fiber 3.0 50 7.0 Comparative Example35 6 6.4 PET fiber 3.0 50 7.0 Comparative Example 36 5 9.5 PET fiber 3.050 7.0 Comparative Example 37 3 12.0 PET fiber 3.0 50 7.0 ComparativeExample 38 3 12.0 PET fiber 2.0 21 2.0 Comparative Example 39 3 12.0 PETfiber 2.0 21 5.0 Comparative Example 40 3 12.0 PET fiber 2.0 21 7.0Comparative Example 41 3 12.0 PET fiber 5.0 10 1.0 Comparative Example42 45 0.7 PET fiber 5.0 10 2.0 Comparative Example 43 45 0.7 PET fiber5.0 10 3.0 Comparative Example 44 3 12.0 PET fiber 5.0 10 3.0Comparative Example 45 45 0.7 PET fiber 5.0 10 5.0 Comparative Example46 3 12.0 PET fiber 5.0 10 7.0 Catalyst coating layer Particle size ofHigh-aspect-ratio pore catalyst particle Rate Catalyst (15% cumulativerelative performance size on to whole NOx Amount of cross-sectionalAverage Porosity Average Orientation of voids conversion coating areabasis) thickness (% by aspect angle (% by efficiency [g/L] [μm] [μm]volume) ratio (degrees) volume) [%] Comparative Example 24 100 13.0 89.878.2 40.2 12.1 65.12 63.9 Comparative Example 25 100 6.9 67.0 71.0 52.49.0 55.42 74.5 Comparative Example 26 100 13.0 95.5 79.2 45.1 20.1 60.1062.9 Comparative Example 27 100 3.4 33.2 54.1 53.3 6.9 56.63 72.3Comparative Example 28 100 6.9 67.0 72.2 51.3 8.1 58.56 73.4 ComparativeExample 29 100 9.7 86.9 78.1 54.2 9.4 52.11 70.5 Comparative Example 30100 3.4 36.8 56.1 54.0 6.7 72.20 70.7 Comparative Example 31 100 5.461.8 70.0 51.9 6.7 62.39 72.2 Comparative Example 32 100 13.0 99.6 81.040.1 10.6 61.00 62.5 Comparative Example 33 100 3.4 34.1 55.2 51.0 6.065.52 70.5 Comparative Example 34 100 5.4 61.3 69.5 52.8 6.8 58.63 69.3Comparative Example 35 100 6.9 68.6 72.9 52.5 7.2 66.65 69.3 ComparativeExample 36 100 9.7 92.7 79.2 58.2 6.9 59.47 66.8 Comparative Example 37100 13.0 99.0 81.6 50.0 9.6 51.00 60.6 Comparative Example 38 100 13.092.7 79.4 15.0 32.1 10.21 64.0 Comparative Example 39 100 13.0 95.5 79.716.4 28.3 12.21 63.3 Comparative Example 40 100 13.0 99.0 80.7 15.4 25.115.21 65.2 Comparative Example 41 100 13.0 92.8 79.7 6.1 60.2 0.40 65.2Comparative Example 42 100 0.9 28.3 49.2 8.2 37.6 54.76 62.6 ComparativeExample 43 100 0.9 31.7 53.4 8.4 38.0 67.29 64.0 Comparative Example 44100 13.0 92.9 80.2 6.5 40.5 0.30 65.8 Comparative Example 45 100 0.934.3 54.0 9.8 35.8 68.92 62.6 Comparative Example 46 100 13.0 98.3 80.07.1 42.0 10.30 61.6

TABLE 5 Oxide particle preparation step Fibrous organic substance ororganic substance Stirring Metal oxide particle Average treatmentParticle size fiber diameter Amount Treatment (50% cumulative or averageAverage mixed time size on volume Type of diameter aspect (parts (min)basis) [μm] raw material [μm] ratio by mass) Comparative Example 47 450.7 Rod-like cellulose 30.0 3 1.0 Comparative Example 48 31 1.9 Rod-likecellulose 30.0 3 1.0 Comparative Example 49 24 3.2 Rod-like cellulose30.0 3 1.0 Comparative Example 50 12 6.4 Rod-like cellulose 30.0 3 1.0Comparative Example 51 3 9.5 Rod-like cellulose 30.0 3 1.0 ComparativeExample 52 3 12.0 Rod-like cellulose 30.0 3 1.0 Comparative Example 5345 0.7 Rod-like cellulose 30.0 3 2.0 Comparative Example 54 24 3.2Rod-like cellulose 30.0 3 2.0 Comparative Example 55 16 4.6 Rod-likecellulose 30.0 3 2.0 Comparative Example 56 12 6.4 Rod-like cellulose30.0 3 2.0 Comparative Example 57 5 9.5 Rod-like cellulose 30.0 3 2.0Comparative Example 58 45 0.7 Rod-like cellulose 30.0 3 3.0 ComparativeExample 59 31 1.9 Rod-like cellulose 30.0 3 3.0 Comparative Example 6024 3.2 Rod-like cellulose 30.0 3 3.0 Comparative Example 61 12 6.4Rod-like cellulose 30.0 3 3.0 Comparative Example 62 5 9.5 Rod-likecellulose 30.0 3 3.0 Comparative Example 63 3 12.0 Rod-like cellulose30.0 3 3.0 Comparative Example 64 45 0.7 Rod-like cellulose 30.0 3 5.0Comparative Example 65 31 1.9 Rod-like cellulose 30.0 3 5.0 ComparativeExample 66 24 3.2 Rod-like cellulose 30.0 3 5.0 Comparative Example 6716 4.6 Rod-like cellulose 30.0 3 5.0 Comparative Example 68 12 6.4Rod-like cellulose 30.0 3 5.0 Comparative Example 69 5 9.5 Rod-likecellulose 30.0 3 5.0 Catalyst coating layer Particle size ofHigh-aspect-ratio pore catalyst particle Rate Catalyst (15% cumulativerelative performance size on to whole NOx Amount of cross-sectionalAverage Porosity Average Orientation of voids conversion coating areabasis) thickness (% by aspect angle (% by efficiency [g/L] [μm] [μm]volume) ratio (degrees) volume) [%] Comparative Example 47 100 0.7 18.940.0 5.1 83.0 0.02 62.3 Comparative Example 48 100 1.9 21.5 43.4 5.271.0 0.02 65.7 Comparative Example 49 100 3.2 29.5 50.3 5.5 74.0 0.0471.0 Comparative Example 50 100 6.4 65.8 72.1 5.9 61.0 0.07 72.9Comparative Example 51 100 9.5 83.7 76.6 5.7 63.0 0.09 67.7 ComparativeExample 52 100 12.0 91.5 79.9 6.1 69.0 0.11 63.5 Comparative Example 53100 0.7 26.5 48.0 5.0 81.0 0.01 62.7 Comparative Example 54 100 3.2 37.856.8 5.5 83.0 0.04 69.8 Comparative Example 55 100 4.6 57.0 67.8 5.774.0 0.06 78.7 Comparative Example 56 100 6.4 73.5 73.8 5.8 78.0 0.0871.4 Comparative Example 57 100 9.5 90.6 78.9 5.7 60.0 0.10 52.8Comparative Example 58 100 0.7 33.6 54.7 5.0 60.0 0.02 65.4 ComparativeExample 59 100 1.9 34.0 54.7 5.2 63.0 0.02 69.3 Comparative Example 60100 3.2 47.0 62.7 5.5 81.0 0.04 71.3 Comparative Example 61 100 6.4 82.576.1 5.8 85.0 0.09 56.2 Comparative Example 62 100 9.5 96.5 80.4 5.762.0 0.09 53.0 Comparative Example 63 100 12.0 114.7 84.3 6.1 78.0 0.1259.4 Comparative Example 64 100 0.7 42.6 59.9 5.1 69.0 0.01 61.0Comparative Example 65 100 1.9 47.4 63.4 5.2 63.0 0.03 65.3 ComparativeExample 66 100 3.2 52.5 65.7 5.6 79.0 0.04 55.2 Comparative Example 67100 4.6 74.0 74.1 5.7 67.0 0.06 74.0 Comparative Example 68 100 6.4 89.178.5 5.9 79.0 0.09 56.8 Comparative Example 69 100 9.5 105.5 82.3 5.780.0 0.10 53.4

TABLE 6 Oxide particle preparation step Fibrous organic substance ororganic substance Stirring Metal oxide particle Average treatmentParticle size fiber diameter Amount Treatment (50% cumulative or averageAverage mixed time size on volume Type of diameter aspect (parts (min)basis) [μm] raw material [μm] ratio by mass) Comparative Example 70 312.0 Rod-like cellulose 30.0 3 5.0 Comparative Example 71 45 0.7Rod-like cellulose 30.0 3 7.0 Comparative Example 72 24 3.2 Rod-likecellulose 30.0 3 7.0 Comparative Example 73 16 4.6 Rod-like cellulose30.0 3 7.0 Comparative Example 74 12 6.4 Rod-like cellulose 30.0 3 7.0Comparative Example 75 45 0.7 Pulverized cellulose 1.0 3 1.0 ComparativeExample 76 31 1.9 Pulverized cellulose 1.0 3 1.0 Comparative Example 7724 3.2 Pulverized cellulose 1.0 3 1.0 Comparative Example 78 16 4.6Pulverized cellulose 1.0 3 1.0 Comparative Example 79 12 6.4 Pulverizedcellulose 1.0 3 1.0 Comparative Example 80 5 9.5 Pulverized cellulose1.0 3 1.0 Comparative Example 81 3 12.0 Pulverized cellulose 1.0 3 1.0Comparative Example 82 45 0.7 Pulverized cellulose 1.0 3 2.0 ComparativeExample 83 24 3.2 Pulverized cellulose 1.0 3 2.0 Comparative Example 8412 6.4 Pulverized cellulose 1.0 3 2.0 Comparative Example 85 5 9.5Pulverized cellulose 1.0 3 2.0 Comparative Example 86 45 0.7 Pulverizedcellulose 1.0 3 3.0 Comparative Example 87 31 1.9 Pulverized cellulose1.0 3 3.0 Comparative Example 88 24 3.2 Pulverized cellulose 1.0 3 3.0Comparative Example 89 16 4.6 Pulverized cellulose 1.0 3 3.0 ComparativeExample 90 12 6.4 Pulverized cellulose 1.0 3 3.0 Comparative Example 915 9.5 Pulverized cellulose 1.0 3 3.0 Comparative Example 92 3 12.0Pulverized cellulose 1.0 3 3.0 Catalyst coating layer Particle size ofHigh-aspect-ratio pore catalyst particle Rate Catalyst (15% cumulativerelative performance size on to whole NOx Amount of cross-sectionalAverage Porosity Average Orientation of voids conversion coating areabasis) thickness (% by aspect angle (% by efficiency [g/L] [μm] [μm]volume) ratio (degrees) volume) [%] Comparative Example 70 100 12.0118.6 85.0 6.0 72.0 0.12 58.0 Comparative Example 71 100 0.7 47.1 63.15.1 76.0 0.02 60.5 Comparative Example 72 100 3.2 57.7 68.8 5.5 79.00.05 54.4 Comparative Example 73 100 4.6 79.0 74.9 5.6 81.0 0.07 70.8Comparative Example 74 100 6.4 86.9 78.2 5.9 60.0 0.08 56.1 ComparativeExample 75 100 0.7 20.4 41.2 5.4 84.0 0.01 67.4 Comparative Example 76100 1.9 21.7 43.3 5.4 82.0 0.02 68.3 Comparative Example 77 100 3.2 30.752.3 6.0 84.0 0.05 76.9 Comparative Example 78 100 4.6 52.4 66.2 5.660.0 0.06 73.7 Comparative Example 79 100 6.4 64.0 70.1 6.0 63.0 0.0873.1 Comparative Example 80 100 9.5 82.0 77.0 5.7 76.0 0.10 72.4Comparative Example 81 100 12.0 92.4 79.5 6.3 62.0 0.11 63.2 ComparativeExample 82 100 0.7 27.9 49.6 5.0 69.0 0.01 78.1 Comparative Example 83100 3.2 31.7 52.2 5.6 83.0 0.05 78.2 Comparative Example 84 100 6.4 68.873.2 5.9 76.0 0.09 73.5 Comparative Example 85 100 9.5 85.2 78.3 6.170.0 0.10 72.4 Comparative Example 86 100 0.7 49.4 63.6 5.0 78.0 0.0178.9 Comparative Example 87 100 1.9 33.8 54.1 5.4 64.0 0.03 73.4Comparative Example 88 100 3.2 36.6 56.3 5.7 67.0 0.05 75.1 ComparativeExample 89 100 4.6 54.5 66.2 6.1 77.0 0.06 72.6 Comparative Example 90100 6.4 70.7 72.9 6.1 68.0 0.08 73.2 Comparative Example 91 100 9.5 88.378.8 6.2 77.0 0.09 70.6 Comparative Example 92 100 12.0 97.9 81.0 6.180.0 0.11 63.3

TABLE 7 Oxide particle preparation step Fibrous organic substance ororganic substance Metal oxide particle Average Stirring Particle sizefiber diameter Amount treatment (50% cumulative or average Average mixedTreatment time size on volume Type of diameter aspect (parts (min)basis) [μm] raw material [μm] ratio by mass) Comparative Example 93 450.7 Pulverized cellulose 1.0 3 5.0 Comparative Example 94 24 3.2Pulverized cellulose 1.0 3 5.0 Comparative Example 95 16 4.6 Pulverizedcellulose 1.0 3 5.0 Comparative Example 96 45 0.7 Pulverized cellulose1.0 3 7.0 Comparative Example 97 31 1.9 Pulverized cellulose 1.0 3 7.0Comparative Example 98 24 3.2 Pulverized cellulose 1.0 3 7.0 ComparativeExample 99 16 6.4 Pulverized cellulose 1.0 3 7.0 Comparative Example 1005 9.5 Pulverized cellulose 1.0 3 7.0 Comparative Example 101 3 12.0Pulverized cellulose 1.0 3 7.0 Comparative Example 102 45 0.7 PMMAparticle 3.0 — 1.0 Comparative Example 103 31 1.9 PMMA particle 3.0 —1.0 Comparative Example 104 24 3.2 PMMA particle 3.0 — 1.0 ComparativeExample 105 16 4.6 PMMA particle 3.0 — 1.0 Comparative Example 106 126.4 PMMA particle 3.0 — 1.0 Comparative Example 107 5 9.5 PMMA particle3.0 — 1.0 Comparative Example 108 3 12.0 PMMA particle 3.0 — 1.0Comparative Example 109 45 0.7 PMMA particle 3.0 — 2.0 ComparativeExample 110 24 3.2 PMMA particle 3.0 — 2.0 Comparative Example 111 126.4 PMMA particle 3.0 — 2.0 Comparative Example 112 45 0.7 PMMA particle3.0 — 3.0 Comparative Example 113 24 3.2 PMMA particle 3.0 — 3.0Comparative Example 114 16 4.6 PMMA particle 3.0 — 3.0 ComparativeExample 115 12 6.4 PMMA particle 3.0 — 3.0 Catalyst coating layerParticle size of High-aspect-ratio pore catalyst particle Rate Catalyst(15% cumulative relative performance size on to whole NOx Amount ofcross-sectional Average Porosity Average Orientation of voids conversioncoating area basis) thickness (% by aspect angle (% by efficiency [g/L][μm] [μm] volume) ratio (degrees) volume) [%] Comparative Example 93 1000.7 56.4 67.1 5.4 77.0 0.02 78.7 Comparative Example 94 100 3.2 40.559.5 5.5 73.0 0.04 75.1 Comparative Example 95 100 4.6 64.4 71.1 5.863.0 0.06 71.8 Comparative Example 96 100 0.7 64.4 71.1 5.1 80.0 0.0173.3 Comparative Example 97 100 1.9 56.6 67.8 5.5 80.0 0.02 68.3Comparative Example 98 100 3.2 52.2 65.1 5.6 68.0 0.05 71.4 ComparativeExample 99 100 6.4 69.1 72.0 6.7 80.0 0.08 69.3 Comparative Example 100100 9.5 94.8 80.5 6.5 70.0 0.10 67.6 Comparative Example 101 100 12.093.4 79.7 6.1 71.0 0.12 63.2 Comparative Example 102 100 0.7 20.6 42.15.1 81.0 0.02 61.4 Comparative Example 103 100 1.9 21.2 43.1 5.2 80.00.03 67.0 Comparative Example 104 100 3.2 28.7 50.7 5.5 79.0 0.05 76.0Comparative Example 105 100 4.6 52.7 66.0 5.6 73.0 0.07 75.1 ComparativeExample 106 100 6.4 63.9 71.2 5.8 74.0 0.08 71.6 Comparative Example 107100 9.5 77.3 75.0 5.8 83.0 0.09 69.3 Comparative Example 108 100 12.089.2 77.7 6.0 62.0 0.11 62.8 Comparative Example 109 100 0.7 24.1 46.85.1 75.0 0.01 63.7 Comparative Example 110 100 3.2 30.3 52.2 5.6 78.00.05 75.1 Comparative Example 111 100 6.4 63.1 70.8 5.8 81.0 0.08 71.1Comparative Example 112 100 0.7 23.8 46.1 5.1 81.0 0.02 65.3 ComparativeExample 113 100 3.2 33.7 54.0 5.6 62.0 0.06 77.1 Comparative Example 114100 4.6 52.4 65.2 5.6 65.0 0.06 74.1 Comparative Example 115 100 6.465.5 70.5 5.9 77.0 0.08 73.4

TABLE 8 Oxide particle preparation step Fibrous organic substance ororganic substance Stirring Metal oxide particle Average treatmentParticle size fiber diameter Amount Treatment (50% cumulative size oraverage Average mixed time on volume basis) Type of diameter aspect(parts (min) [μm] raw material [μm] ratio by mass) Comparative Example116 5 9.5 PMMA particle 3.0 — 3.0 Comparative Example 117 45 0.7 PMMAparticle 3.0 — 5.0 Comparative Example 118 31 1.9 PMMA particle 3.0 —5.0 Comparative Example 119 16 4.6 PMMA particle 3.0 — 5.0 ComparativeExample 120 5 9.5 PMMA particle 3.0 — 5.0 Comparative Example 121 3 12.0PMMA particle 3.0 — 5.0 Comparative Example 122 31 1.9 PMMA particle 3.0— 7.0 Comparative Example 123 24 3.2 PMMA particle 3.0 — 7.0 ComparativeExample 124 12 6.4 PMMA particle 3.0 — 7.0 Comparative Example 125 5 9.5PMMA particle 3.0 — 7.0 Comparative Example 126 3 12.0 PMMA particle 3.0— 7.0 Comparative Example 127 31 1.9 PMMA particle + TiOPr + PEG 3.0 111.0 Comparative Example 128 3 12.0 PMMA particle + TiOPr + PEG 3.0 111.0 Comparative Example 129 12 6.4 PMMA particle + TiOPr + PEG 3.0 112.0 Comparative Example 130 3 12.0 PMMA particle + TiOPr + PEG 3.0 113.0 Comparative Example 131 3 12.0 PMMA particle + TiOPr + PEG 3.0 117.0 Comparative Example 132 16 4.6 PET fiber 2.0 40 3.0 ComparativeExample 133 16 4.6 PET fiber 2.0 40 3.0 Catalyst coating layer Particlesize of High-aspect-ratio pore catalyst particle Rate Catalyst (15%cumulative relative performance size on to whole NOx Amount ofcross-sectional Average Porosity Average Orientation of voids conversioncoating area basis) thickness (% by aspect angle (% by efficiency [g/L][μm] [μm] volume) ratio (degrees) volume) [%] Comparative Example 116100 9.5 86.4 77.5 5.8 69.0 0.09 69.5 Comparative Example 117 100 0.725.9 47.2 5.1 65.0 0.01 67.1 Comparative Example 118 100 1.9 27.7 49.05.2 68.0 0.02 72.7 Comparative Example 119 100 4.6 56.8 68.4 5.7 74.00.06 57.8 Comparative Example 120 100 9.5 84.2 78.0 5.7 70.0 0.12 67.5Comparative Example 121 100 12.0 95.0 80.1 6.0 73.0 0.12 63.8Comparative Example 122 100 1.9 30.3 52.3 5.2 81.0 0.02 71.7 ComparativeExample 123 100 3.2 37.1 57.3 5.6 60.0 0.05 78.2 Comparative Example 124100 6.4 69.2 72.7 5.9 85.0 0.09 68.0 Comparative Example 125 100 9.585.9 77.0 5.8 79.0 0.10 63.8 Comparative Example 126 100 12.0 97.6 80.26.0 85.0 0.11 59.9 Comparative Example 127 100 1.9 21.6 45.0 5.5 46.90.37 69.8 Comparative Example 128 100 12.0 92.3 79.4 5.1 69.0 0.20 62.3Comparative Example 129 100 6.4 63.2 69.8 8.9 47.0 0.42 73.5 ComparativeExample 130 100 12.0 98.6 80.1 6.1 65.0 0.51 69.1 Comparative Example131 100 12.0 93.9 79.2 5.4 42.0 0.41 79.0 Comparative Example 132 30 4.622.0 66.9 41.1 20.1 32.21 75.0 Comparative Example 133 400 4.6 181.670.4 43.6 26.4 46.10 72.02. Evaluation

The exhaust gas purification catalyst (catalyst sample) obtained in eachof Examples 1 to 42 and the comparative exhaust gas purificationcatalyst (comparative catalyst sample) obtained in each of ComparativeExamples 1 to 133 were subjected to measurements of the averagethickness [μm] of the catalyst coating layer, the particle size of thecatalyst particle (15% cumulative size on cross-sectional area basis)[μm], the porosity [% by volume] of the catalyst coating layer, theaverage aspect ratio of the high-aspect-ratio pore, the rate [%] of thehigh-aspect-ratio pore relative to the whole of a void, and theorientation angle [degrees)(° ] of the high-aspect-ratio pore (80%cumulative angle).

(1) Measurement Test of Average Thickness of Catalyst Coating Layer

Each of the catalyst sample and the comparative catalyst sample wasembedded with an epoxy resin and cut out in a radial direction of thesubstrate (honeycomb-shaped substrate), and the resulting cross sectionwas polished. The resultant was subjected to scanning electronmicroscope (SEM) observation (magnification: 700-fold) to measure theaverage thickness of the catalyst coating layer. Herein, the averagethickness was determined by randomly extracting 10 points on thecatalyst coating layer, and measuring the thickness of the catalystcoating layer at such points to provide an average value. The resultsobtained are shown in Table 1 to Table 8.

(2) Measurement Test of Particle Size of Catalyst Particle

Each of the catalyst sample and the comparative catalyst sample wasembedded with an epoxy resin and cut out in a radial direction of thesubstrate (honeycomb-shaped substrate), and the resulting cross sectionwas polished. The resultant was subjected to scanning electronmicroscope (SEM) observation (magnification: 700-fold) to determine the15% cumulative size in a cumulative particle size distribution on across-sectional area basis of the catalyst particle. Herein, the 15%cumulative size of the particle size of catalyst particle on across-sectional area basis was determined as follows: the catalystparticles in a square region of 200 μm or more in a horizontal directionto a substrate flat portion and 25 μm or more in a perpendiculardirection to the substrate flat portion, of the catalyst coating layer,were extracted; and the particle size of the catalyst particle wasmeasured which corresponded to the particle size at 15% in terms offrequency relative to the whole of the cross-sectional area of thecatalyst coating layer when the cross-sectional area of the catalystparticle was cumulated from the largest catalyst particle size(cross-sectional area) of the catalyst particle in the descending order,provided that any pore where the sum of the cross-sectional area of thecatalyst particle was less than 0.3 μm² was excluded. The resultsobtained are shown in Table 1 to Table 8.

(3) Measurement Test of Porosity of Catalyst Coating Layer

The porosity of the catalyst sample was measured based on the followingexpression by a weight-in-water method, according to JIS R 2205. Herein,deairing was performed by vacuum deairing.Porosity (% by volume)=(W3−W1)/(W3−W2)×100W1: dry mass (120° C.×60 minutes)W2: mass in waterW3: mass in saturation with water

The results obtained are shown in Table 1 to Table 8.

(4) Measurement Test 1 of Pore in Catalyst Coating Layer: EquivalentCircle Diameter of Pore

The pore in the catalyst coating layer of each of the catalyst sampleand the comparative catalyst sample was subjected to FIB-SEM analysis.

First, each of the catalyst sample and the comparative catalyst samplewas cut out in an axial direction at the position of a dotted lineillustrated in FIG. 1(A), to provide a test piece having a shapeillustrated in FIG. 2(B). Next, while a range of the test piece,indicated by a dotted line of a square frame in FIG. 1(B), was cut byFIB (focused ion beam machining apparatus, manufactured by HitachiHigh-Technologies Corporation, trade name “NB5000”), a SEM (scanningelectron microscope, manufactured by Hitachi High-TechnologiesCorporation, trade name “NB5000”) image was taken in a pitch of 0.28 μmin a depth illustrated in FIG. 1(C). Herein, FIB-SEM analysis conditionswere as follows: the length was 25 μm or more, the width was 500 μm ormore, the depth in measurement was 500 μm or more, the number of viewsin imaging was 3 or more, and the imaging magnification was 2000-fold inthe SEM image. FIG. 1 includes schematic diagrams illustrating oneexample of a FIB-SEM measurement method. FIG. 1(A) is a schematicdiagram illustrating a part of a catalyst coating layer cross sectionperpendicular to an exhaust gas flow direction of the substrate of theexhaust gas purification catalyst of the present invention, FIG. 1(B) isa schematic diagram illustrating a test piece obtained by cutting theexhaust gas purification catalyst in an axial direction at the positionof a dotted line illustrated in FIG. 1(A), and FIG. 1(C) schematicallyrepresents an SEM image obtained by a FIB-SEM measurement method. FIG. 2illustrates one continuous cross-sectional SEM image of the catalystsample in Example 5 subjected to measurement, as one example of theobservation results of FIB-SEM analysis. A black portion in FIG. 2represents a pore. FIG. 2 illustrates a scanning electron micrograph(SEM photograph) of a catalyst coating layer cross section perpendicularto an exhaust gas flow direction of the substrate of the exhaust gaspurification catalyst obtained in Example 5. Herein, a continuous imageas illustrated in FIG. 1(c) can be taken by X-ray CT or the like.

Next, the continuous cross-sectional image (SEM image) obtained byFIB-SEM analysis was subjected to image analysis using commerciallyavailable image analysis software (manufactured by Mitani Corporation,“two-dimensional image analysis software WinROOF”) by means of thedifference in brightness between the pore and the catalyst, and wassubjected to binarization processing to extract the pore. FIG. 3illustrates the SEM photograph in FIG. 2, subjected to binarizationprocessing, as one example of the results obtained. In FIG. 3, a blackportion represents the catalyst, and a white portion represents thepore. In the pore analysis, a pore whose equivalent circle diameter inthe cross-sectional image of a catalyst coating layer cross sectionperpendicular to an exhaust gas flow direction of the substrate was 2 μmor more was analyzed. In addition, the function for extracting a subjectby use of the difference in brightness is not limited to WinROOF, andsuch function (for example, image-Pro Plus manufactured by Planetron,Inc.) on which common analysis software is normally mounted can beutilized.

The area within the profile of the pore was determined by such imageanalysis, the equivalent circle diameter of the pore was calculated, andthe equivalent circle diameter as the particle size of the pore wasobtained.

(5) Measurement Test 2 of Pore in Catalyst Coating Layer: Average AspectRatio of High-Aspect-Ratio Pore

Next, the continuous cross-sectional image obtained by the above methodwas analyzed, and three-dimensional information on the pore wasextracted. The measurement method of the average aspect ratio of thehigh-aspect-ratio pore was the same as the method described withreference to FIG. 4 and FIG. 5 described above, and the average aspectratio of the high-aspect-ratio pore was determined by creating thetwo-dimensional projection diagram and the cross-sectional image of thepore exemplifying the three-dimensional information on the porecorresponding to that in FIG. 4 and FIG. 5, and analyzing a SEM imagewithin a length of 25 μm or more, a width of 500 μm or more and a depthin measurement of 500 μm or more, of the high-aspect-ratio pore (thenumber of views in imaging was 3 or more, and the imaging magnificationwas 2000-fold). Herein, the two-dimensional projection diagramexemplifying the three-dimensional information on the pore, obtained byanalyzing the continuous cross-sectional image of the catalyst coatinglayer cross section perpendicular to an exhaust gas flow direction ofthe substrate of the exhaust gas purification catalyst obtained inExample 5 was the same as the two-dimensional projection diagramexemplifying the three-dimensional information on the pore illustratedin FIG. 4. As a result, the average aspect ratio of thehigh-aspect-ratio pore in Example 5 was 18.9. In addition, Table 1 toTable 8 show measurement results (the average aspect ratio of thehigh-aspect-ratio pore) of Examples other than Example 5, andComparative Examples.

(6) Measurement Test 3 of Pore in Catalyst Coating Layer: Rate ofHigh-Aspect-Ratio Pore Relative to Whole of Voids

Next, the rate of the high-aspect-ratio pore relative to the whole of avoid was determined by dividing the porosity of the high-aspect-ratiopore by the porosity of the catalyst coating layer.

Herein, the porosity (% by volume) of the high-aspect-ratio pore wascalculated by first extracting the high-aspect-ratio pore in a SEM image(the number of views in imaging was 3 or more, and the imagingmagnification was 2000-fold) within a length of 25 μm or more, a widthof 500 μm or more, and a depth in measurement of 500 μm or more, andcalculating the volume of each pore according to the following method.In other words, the volume of the high-aspect-ratio pore was calculatedby multiplying a pitch of the continuous cross-sectional image, of 0.28μm, with the area of the cross section of the high-aspect-ratio pore inthe cross-sectional image obtained by FIB-SEM, and integrating such aproduct. Next, the resulting “volume of the high-aspect-ratio pore” wasdivided by the volume of the range subjected to imaging by FIB-SEM (theSEM image range), thereby providing the porosity (% by volume) of thehigh-aspect-ratio pore.

Next, the rate (% by volume) of the high-aspect-ratio pore relative tothe whole of a void was determined by dividing the porosity (% byvolume) of the resulting high-aspect-ratio pore by the porosity (% byvolume) of the catalyst coating layer obtained in the “Measurement testof porosity of catalyst coating layer” (“Rate of high-aspect-ratio porerelative to whole of voids (%)”=“porosity of high-aspect-ratio pore (%by volume)”/“porosity of catalyst coating layer (% by volume)”×100).

As a result, the rate of the high-aspect-ratio pore relative to thewhole of a void in Example 5 was 11.1% by volume. In addition, Table 1to Table 8 show measurement results (the rate of the high-aspect-ratiopore relative to the whole of a void) of Examples other than Example 5,and Comparative Examples.

(7) Measurement Test 4 of Pore in Catalyst Coating Layer: OrientationAngle of High-Aspect-Ratio Pore

Next, the 80% cumulative angle, in a cumulative angle distribution on anangle basis, of the angle (cone angle) between a vector in alongitudinal direction of the high-aspect-ratio pore and a vector in anexhaust gas flow direction of the substrate was determined as theorientation angle of the high-aspect-ratio pore. Here, the measurementmethod of the orientation angle (80% cumulative angle) of thehigh-aspect-ratio pore was the same as the method with reference to FIG.4 to FIG. 6 described above. Herein, the two-dimensional projectiondiagram obtained in Example 5 is the same as the two-dimensionalprojection diagram exemplified in FIG. 4, and FIG. 6 is the same as theschematic diagram illustrating the cone angle of the high-aspect-ratiopore in the two-dimensional projection diagram obtained in Example 5. Asillustrated in the schematic diagram of FIG. 6, the angle (cone angle)between the vector (Y) in a longitudinal direction of thehigh-aspect-ratio pore and the vector (X) in an exhaust gas flowdirection (axial direction of honeycomb) of the substrate wasdetermined, and the 80% cumulative angle, in a cumulative angledistribution on an angle basis, of the cone angle was calculated byimage analysis of the three-dimensional image. Herein, the orientationangle (80% cumulative angle) of the high-aspect-ratio pore wasdetermined by randomly extracting 20 of the high-aspect-ratio pores, andmeasuring the 80% cumulative angle, in a cumulative angle distributionon an angle basis, of the cone angle of each of the high-aspect-ratiopores to provide an average value. Table 1 to Table 8 show respectiveresults obtained (80% cumulative angle).

(8) Catalyst Performance Evaluation Test

The catalyst sample obtained in each of Examples 1 to 42 and ComparativeExamples 1 to 133 was subjected to a NOx conversion efficiencymeasurement test as described below, and the catalyst performance ofeach catalyst was evaluated.

(NOx Conversion Efficiency Measurement Test)

The catalyst sample obtained in each of Examples 1 to 42 and ComparativeExamples 1 to 133 was subjected to NOx conversion efficiency measurementin an atmosphere in transient variation during a transient period, asdescribed below.

That is, a straight four-cylinder 2.4-L engine was used to perform A/Ffeedback control as targets of 14.1 and 15.1, first, and the NOxconversion efficiency was calculated from the average amount of NOxdischarged in A/F switching. The engine operation conditions and thesetup of piping were adjusted so that the intake air mass was 40 (g/sec)and the temperature of gas flowing into the catalyst was 750° C.

3. Results

(1) Relationship Between Amount of Coating of Catalyst Coating Layer andCatalyst Performance

FIG. 7 illustrates a graph representing a relationship between theamount of coating of the catalyst coating layer and the NOx conversionefficiency as a graph representing catalyst performance evaluation testresults of each catalyst obtained in Examples 1 to 42 and ComparativeExamples 1 to 133. As is clear from comparison of the results inExamples 1 to 42 and the results in Comparative Examples 1 to 133illustrated in FIG. 7 and represented in Table 1 to Table 8, it wasconfirmed that the exhaust gas purification catalyst in each of Examples1 to 42 exhibited excellent catalyst performance in an amount of coatingof the catalyst coating layer, ranging from 50 to 300 g/L, even in aregion under a high load with a high flow rate of gas.

(2) Relationship Between Average Thickness of Catalyst Coating Layer andCatalyst Performance

FIG. 8 illustrates a graph representing a relationship between theaverage thickness of the catalyst coating layer and the NOx conversionefficiency as a graph representing catalyst performance evaluation testresults of each catalyst obtained in Examples 1 to 42 and ComparativeExamples 1 to 133. As is clear from comparison of the results inExamples 1 to 42 and the results in Comparative Examples 1 to 133illustrated in FIG. 8 and represented in Table 1 to Table 8, it wasconfirmed that the exhaust gas purification catalyst in each of Examples1 to 42 exhibited excellent catalyst performance at an average thicknessof the catalyst coating layer, ranging from 25 μm to 160 μm, even in aregion under a high load with a high flow rate of gas.

(3) Relationship Between Particle Size of Catalyst Particle and CatalystPerformance

FIG. 9 illustrates a graph representing a relationship between theparticle size of the catalyst particle (the 15% cumulative size, in acumulative particle size distribution on a cross-sectional area basis,of the catalyst particle) and the NOx conversion efficiency as a graphrepresenting catalyst performance evaluation test results of eachcatalyst obtained in Examples 1 to 42 and Comparative Examples 1 to 133.As is clear from comparison of the results in Examples 1 to 42 and theresults in Comparative Examples 1 to 133 illustrated in FIG. 9 andrepresented in Table 1 to Table 8, it was confirmed that the exhaust gaspurification catalyst in each of Examples 1 to 42 exhibited excellentcatalyst performance in a particle size of the catalyst particle (15%cumulative size on cross-sectional area basis), ranging from 3 to 10 μm,even in a region under a high load with a high flow rate of gas.

(4) Relationship Between Porosity of Catalyst Coating Layer and CatalystPerformance

FIG. 10 illustrates a graph representing a relationship between theporosity of the catalyst coating layer (the porosity measured by aweight-in-water method) and the NOx conversion efficiency as a graphrepresenting catalyst performance evaluation test results of eachcatalyst obtained in Examples 1 to 42 and Comparative Examples 1 to 133.As is clear from comparison of the results in Examples 1 to 42 and theresults in Comparative Examples 1 to 133 illustrated in FIG. 10 andrepresented in Table 1 to Table 8, it was confirmed that the exhaust gaspurification catalyst in each of Examples 1 to 42 exhibited excellentcatalyst performance in a porosity of the catalyst coating layer,ranging from 50 to 80% by volume, even in a region under a high loadwith a high flow rate of gas.

(5) Relationship Between Average Aspect Ratio of High-Aspect-Ratio Poreand Catalyst Performance

First, FIG. 11 represents a graph representing a relationship betweenthe aspect ratio (determined by analyzing any pore where the equivalentcircle diameter of the pore in a cross-sectional image of a catalystcoating layer cross section perpendicular to an exhaust gas flowdirection of the substrate was 2 μm or more, and corresponding to theaspect ratio of the high-aspect-ratio pore having an aspect ratio of 5or more among the pores determined) and the frequency (%) of thehigh-aspect-ratio pore of the catalyst obtained in Example 5. Herein,FIG. 11 also represents a relationship between the aspect ratio and thefrequency (%) of the pore of the catalyst obtained in ComparativeExample 4. It was confirmed from comparison of the result in Example 5and the result in Comparative Example 4 illustrated in FIG. 11 that thecomparative exhaust gas purification catalyst in Comparative Example 4was very few in the number of the high-aspect-ratio pore.

Next, FIG. 12 illustrates a graph representing a relationship betweenthe average aspect ratio (determined by analyzing any pore where theequivalent circle diameter of the pore in a cross-sectional image of acatalyst coating layer cross section perpendicular to an exhaust gasflow direction of the substrate was 2 μm or more, and corresponding tothe average aspect ratio of the high-aspect-ratio pore having an aspectratio of 5 or more among the pores determined) and the NOx conversionefficiency of the high-aspect-ratio pore, as a graph representingcatalyst performance evaluation test results of each catalyst obtainedin Examples 1 to 42 and Comparative Examples 1 to 133. As is clear fromcomparison of the results in Examples 1 to 42 and the results inComparative Examples 1 to 133 illustrated in FIG. 12 and represented inTable 1 to Table 8, it was confirmed that the exhaust gas purificationcatalyst in each of Examples 1 to 42 exhibited excellent catalystperformance in an average aspect ratio of the high-aspect-ratio pore,ranging from 10 to 50, even in a region under a high load with a highflow rate of gas.

(6) Relationship Between Rate of High-Aspect-Ratio Pore Relative toWhole of Voids and Catalyst Performance

FIG. 13 illustrates a graph representing a relationship between the rateof the high-aspect-ratio pore relative to the whole of a void (the rateof the high-aspect-ratio pore) and the NOx conversion efficiency, as agraph representing catalyst performance evaluation test results of eachcatalyst obtained in Examples 1 to 42 and Comparative Examples 1 to 133.As is clear from comparison of the results in Examples 1 to 42 and theresults in Comparative Examples 1 to 133 illustrated in FIG. 13 andrepresented in Table 1 to Table 8, it was confirmed that the exhaust gaspurification catalyst in each of Examples 1 to 42 exhibited excellentcatalyst performance at a rate of the high-aspect-ratio pore relative tothe whole of a void, ranging from 0.5 to 50%, even in a region under ahigh load with a high flow rate of gas.

(7) Relationship Between 80% Cumulative Angle of High-Aspect-Ratio Poreand Catalyst Performance

First, FIG. 14 illustrates a graph representing a relationship betweenthe cone angle (degrees (°), the angle formed by vector Y in alongitudinal direction of the high-aspect-ratio pore and vector X in anexhaust gas flow direction of the substrate) and the cumulative rate (%)of the high-aspect-ratio pore of the catalyst obtained in Example 16. Itwas confirmed from FIG. 14 that the cone angle had a distribution.

Next, FIG. 15 illustrates a graph representing a relationship betweenthe 80% cumulative angle (the 80% cumulative angle in a cumulative angledistribution on an angle basis of the angle (cone angle) formed byvector Y in a longitudinal direction of the high-aspect-ratio pore andvector X in an exhaust gas flow direction of the substrate) and the NOxconversion efficiency of the high-aspect-ratio pore, as a graphrepresenting catalyst performance evaluation test results of eachcatalyst obtained in Examples 1 to 42 and Comparative Examples 1 to 133.As is clear from comparison of the results in Examples 1 to 42 and theresults in Comparative Examples 1 to 133 illustrated in FIG. 15 andrepresented in Table 1 to Table 8, it was confirmed that the exhaust gaspurification catalyst in each of Examples 1 to 42 exhibited excellentcatalyst performance at an orientation angle (the 80% cumulative angleon an angle basis) of the high-aspect-ratio pore, ranging from 0 to 45degrees (°), even in a region under a high load with a high flow rate ofgas.

It has been confirmed from the above results that the exhaust gaspurification catalyst of the present invention is an exhaust gaspurification catalyst which can exhibit excellent catalyst performanceeven in a region under a high load with a high flow rate of gas. [Test2: Preparation and evaluation of exhaust gas purification catalystcomprising dual catalyst of combination of S/C and UF/C]

1. Preparation of Catalyst

(1) Comparative Example 1: exhaust gas purification catalyst comprisingdual catalyst of combination of S/C prepared using no pore-formingmaterial and UF/C prepared using no pore-forming material

(a) S/C prepared using no pore-forming material

(a1) Lower Pd layer (Pd(1.0)/CZ(50)+Al₂O₃(75))

An aqueous palladium nitrate solution (produced by Cataler Corporation)having a noble metal content of 8.8% by weight was used, and a Pd/CZmaterial where Pd was supported on a ceria zirconia composite oxidematerial (composite oxide made of 30% by weight of CeO₂, 60% by weightof ZrO₂, 5% by weight of Y₂O₃ and 5% by weight of La₂O₃: hereinafter,referred to as “CZ material”) was prepared by an impregnation method.Next, the Pd/CZ material and a composite Al₂O₃ carrier containing 1% byweight of La₂O₃, and an Al₂O₃-based binder were added to and suspendedin distilled water with stirring, thereby preparing slurry 1. The 15%cumulative size in a cumulative particle size distribution on across-sectional area of the particle included in the slurry was 3.3 μm.

Slurry 1 was allowed to flow into a cordierite honeycomb structuresubstrate (600H/3-9R-08, manufactured by Denso Corporation) having avolume of 875 cc, and an unnecessary content was then blown off by ablower to coat the wall surface of the substrate. The coating was formedso as to include 1.0 g/L of Pd, 75 g/L of a composite Al₂O₃ carrier and50 g/L of a Pd/CZ material based on the volume of the substrate. Afterthe coating was formed, the water content was removed in a dryer at 120°C. for 2 hours, and thereafter the resultant was fired in an electricfurnace at 500° C. for 2 hours. The thickness of the coating based onSEM observation was 35 μm, and the porosity of the coating based on aweight-in-water method was 73%.

(a2) Upper Rh layer (Rh(0.2)/CZ(60)+Al₂O₃(40))

An aqueous rhodium hydrochloride solution (produced by CatalerCorporation) having a noble metal content of 2.8% by weight was used,and a Rh/CZ material where Rh was supported on a CZ material wasprepared by an impregnation method. Next, the Rh/CZ material and acomposite Al₂O₃ carrier containing 1% by weight of La₂O₃, and anAl₂O₃-based binder were added to and suspended in distilled water withstirring, thereby preparing slurry 2. The 15% cumulative size in acumulative particle size distribution on a cross-sectional area of theparticle included in the slurry was 3.2 μm.

Slurry 2 was allowed to flow into the substrate subjected to coatingaccording to (a1) above, and an unnecessary content was then blown offby a blower to coat the wall surface of the substrate. The coating wasformed so as to include 0.2 g/L of Rh, 40 g/L of a composite Al₂O₃carrier and 60 g/L of a Rh/CZ material based on the volume of thesubstrate. After the coating was formed, the water content was removedin a dryer at 120° C. for 2 hours, and thereafter the resultant wasfired in an electric furnace at 500° C. for 2 hours. The thickness ofthe coating based on SEM observation was 27 μm, and the porosity of thecoating based on a weight-in-water method was 72%.

(b) UF/C prepared using no pore-forming material

(b1) Lower Pt layer (Pt(1.0)/CZ(30)+Al₂O₃(50))

An aqueous platinum nitrate solution (produced by Cataler Corporation)having a noble metal content of 2.8% by weight was used, and a Pt/CZmaterial where Pt was supported on a CZ material was prepared by animpregnation method. Next, the Pt/CZ material and a composite Al₂O₃carrier containing 1% by weight of La₂O₃, and an Al₂O₃-based binder wereadded to and suspended in distilled water with stirring, therebypreparing slurry 3. The 15% cumulative size, in a cumulative particlesize distribution on a cross-sectional area basis, of the particleincluded in the slurry was 3.0 μm.

Slurry 3 was allowed to flow into a cordierite honeycomb structuresubstrate (600H/3-9R-08, manufactured by Denso Corporation) having avolume of 875 cc, and an unnecessary content was then blown off by ablower to coat the wall surface of the substrate. The coating was formedso as to include 1.0 g/L of Pt, 50 g/L of the composite Al₂O₃ carrierand 30 g/L of the Pt/CZ material based on the volume of the substrate.After the coating was formed, the water content was removed in a dryerat 120° C. for 2 hours, and thereafter the resultant was fired in anelectric furnace at 500° C. for 2 hours. The thickness of the coatingbased on SEM observation was 21 μm, and the porosity of the coatingbased on a weight-in-water method was 71%.

(b2) Upper Rh layer (Rh(0.2)/CZ(20)+Al₂O₃(40))

An aqueous rhodium hydrochloride solution (produced by CatalerCorporation) having a noble metal content of 2.8% by weight was used,and a Rh/CZ material where Rh was supported on a CZ material wasprepared by an impregnation method. Next, the Rh/CZ material and acomposite Al₂O₃ carrier containing 1% by weight of La₂O₃, and anAl₂O₃-based binder were added to and suspended in distilled water withstirring, thereby preparing slurry 4. The 15% cumulative size in acumulative particle size distribution on a cross-sectional area basis ofthe particle included in the slurry was 3.1 μm.

Slurry 4 was allowed to flow into the substrate subjected to coatingaccording to (b1) above, and an unnecessary content was then blown offby a blower to coat the wall surface of the substrate. The coating wasformed so as to include 0.2 g/L of Rh, 40 g/L of the composite Al₂O₃carrier and 20 g/L of the Rh/CZ material based on the volume of thesubstrate. After the coating was formed, the water content was removedin a dryer at 120° C. for 2 hours, and thereafter the resultant wasfired in an electric furnace at 500° C. for 2 hours. The thickness ofthe coating based on SEM observation was 19 μm, and the porosity of thecoating based on a weight-in-water method was 69%.

S/C and UF/C prepared in (a) and (b), respectively, were combined tothereby prepare an exhaust gas purification catalyst comprising a dualcatalyst.

(2) Comparative Example 2: exhaust gas purification catalyst comprisingdual catalyst of combination of S/C prepared using pore-forming materialand UF/C prepared using no pore-forming material

A catalyst was prepared in the same manner as in Comparative Example 1except that 3% by weight of a PET fiber having a diameter ((p) of 2 μmand a length (L) of 80 μm based on the weight of the metal oxideparticle was further added as the pore-forming material in preparationof each of slurry 1 and slurry 2.

The 15% cumulative size in a cumulative particle size distribution on across-sectional area basis of the particle included in slurry 1 to whichthe pore-forming material was added was 3.4 μm. In addition, in thelower layer coating formed using slurry 1 to which the pore-formingmaterial was added, the thickness of the coating based on SEMobservation was 37 μm, and the porosity of the coating based on aweight-in-water method was 75%. The volume ratio of thehigh-aspect-ratio pore having an aspect ratio of 5 or more relative tothe whole of a void in the coating was 8%, and the average aspect ratioof the high-aspect-ratio pore was 41 (both were based on 3D measurementby FIB-SEM).

The 15% cumulative size in a cumulative particle size distribution on across-sectional area basis of the particle included in slurry 2 to whichthe pore-forming material was added was 3.0 μm. In the upper layercoating formed using slurry 2 to which the pore-forming material wasadded, the thickness of the coating based on SEM observation was 37 μm,and the porosity of the coating based on a weight-in-water method was76%. The volume ratio of the high-aspect-ratio pore having an aspectratio of 5 or more relative to the whole of a void in the coating was9%, and the average aspect ratio of the high-aspect-ratio pore was 40(both were based on 3D measurement by FIB-SEM).

(3) Comparative Example 3: exhaust gas purification catalyst comprisingdual catalyst of combination of S/C prepared using pore-forming materialand UF/C prepared using pore-forming material

A catalyst was prepared in the same manner as in Comparative Example 1except that 3% by weight of a PET fiber having a diameter ((p) of 2 μmand a length (L) of 80 μm based on the weight of the metal oxideparticle was further added as the pore-forming material in preparationof each of slurry 1, slurry 2, slurry 3 and slurry 4.

The 15% cumulative size in a cumulative particle size distribution on across-sectional area basis of the particle included in slurry 3 to whichthe pore-forming material was added was 3.0 μm. In the lower layercoating formed using slurry 3 to which the pore-forming material wasadded, the thickness of the coating based on SEM observation was 33 μm,and the porosity of the coating based on a weight-in-water method was73%. The volume ratio of high-aspect-ratio pores having an aspect ratioof 5 or more relative to the whole volume of voids in the coating was7%, and the average aspect ratio of the high-aspect-ratio pore was 42(both were based on 3D measurement by FIB-SEM).

The 15% cumulative size in a cumulative particle size distribution on across-sectional area basis of the particle included in slurry 4 to whichthe pore-forming material was added was 3.0 μm. In the upper layercoating formed using slurry 4 to which the pore-forming material wasadded, the thickness of the coating based on SEM observation was 34 μm,and the porosity of the coating based on a weight-in-water method was72%. The volume ratio of high-aspect-ratio pores having an aspect ratioof 5 or more relative to the whole volume of voids in the coating was8%, and the average aspect ratio of the high-aspect-ratio pore was 40(both were based on 3D measurement by FIB-SEM).

(4) Example 1: exhaust gas purification catalyst comprising dualcatalyst of combination of S/C prepared using no pore-forming materialand UF/C prepared using pore-forming material

A catalyst was prepared in the same manner as in Comparative Example 1except that 3% by weight of a PET fiber having a diameter ((p) of 2 μmand a length (L) of 80 μm based on the weight of the metal oxideparticle was further added as the pore-forming material in preparationof each of slurry 3 and slurry 4.

2. Evaluation

(1) Evaluation of NOx purification performance under high Ga conditionduring transient period of A/F

Each catalyst was mounted on a 2AR-FE engine (manufactured by ToyotaMotor Corporation), A/F was actively controlled as targets of 14.1 and15.1, the intake air mass was adjusted to 40 g/s, the temperature of gasflowing into the S/C catalyst was adjusted to 750° C., the temperatureof gas flowing into the UF/C catalyst was adjusted to 450° C., and theNOx purification performance was evaluated.

3. Results

FIG. 16 is a graph representing measurement results of the NOxconversion efficiency under a high Ga condition during the transientperiod of A/F. The catalyst in Example 1, prepared using thepore-forming material only in UF/C, was more excellent in NOxpurification performance under a high Ga condition during the transientperiod of A/F, than the catalyst in each of Comparative Examples 1 to 3.It was presumed therefrom that the gas diffusivity of UF/C having amajor effect on gas diffusivity was enhanced to thereby enhance the useefficiency of an active site of UF/C.

All documents, patents and patent publications cited in the presentdescription are herein incorporated by reference as they are.

The invention claimed is:
 1. An exhaust gas purification catalystcomprising a dual catalyst of a combination of a startup catalyst (S/C)and an underfloor catalyst (UF/C) that is disposed posterior to the S/Cin an exhaust gas flow direction, wherein: a catalyst coating of theUF/C is configured from one or two or more layers; in at least one layerof the catalyst coating of the UF/C, an average thickness of the coatinglayer is in a range from 25 μm to 160 μm, a porosity measured by aweight-in-water method is in a range from 50 to 80% by volume, andhigh-aspect-ratio pores having an aspect ratio of 5 or more account for0.5 to 50% by volume of the whole volume of voids; and thehigh-aspect-ratio pore has an equivalent circle diameter of from 2 μm to50 μm in a cross-sectional image of a catalyst coating layer crosssection perpendicular to an exhaust gas flow direction and has anaverage aspect ratio of from 10 to
 50. 2. The exhaust gas purificationcatalyst according to claim 1, wherein in the catalyst coating of theUF/C, the high-aspect-ratio pore is oriented such that an 80% cumulativeangle, in a cumulative angle distribution on an angle basis, of an angle(cone angle) between a vector in a longitudinal direction of thehigh-aspect-ratio pore and a vector in an exhaust gas flow direction ofa substrate is in a range from 0 to 45 degrees.
 3. The exhaust gaspurification catalyst according to claim 1, wherein a 15% cumulativesize, in a cumulative particle size distribution on a cross-sectionalarea basis, of a catalyst particle contained in the catalyst coating ofthe UF/C is in a range from 3 μm to 10 μm.
 4. The exhaust gaspurification catalyst according to claim 1, wherein in the catalystcoating of the UF/C, an amount of coating is in a range from 50 to 300 gper liter of the volume of a substrate.
 5. The exhaust gas purificationcatalyst according to claim 2, wherein a 15% cumulative size, in acumulative particle size distribution on a cross-sectional area basis,of a catalyst particle contained in the catalyst coating of the UF/C isin a range from 3 μm to 10 μm.
 6. The exhaust gas purification catalystaccording to claim 2, wherein in the catalyst coating of the UF/C, anamount of coating is in a range from 50 to 300 g per liter of the volumeof a substrate.
 7. The exhaust gas purification catalyst according toclaim 3, wherein in the catalyst coating of the UF/C, an amount ofcoating is in a range from 50 to 300 g per liter of the volume of asubstrate.
 8. The exhaust gas purification catalyst according to claim5, wherein in the catalyst coating of the UF/C, an amount of coating isin a range from 50 to 300 g per liter of the volume of a substrate.